Strontium accumulation by sarcoplasmic reticulum and mitochondria in vascular smooth muscle

Cell-To-Cell Communication in the Resistance Vasculature

The arterial vasculature can be divided into large conduit arteries, intermediate contractile arteries, resistance arteries, arterioles, and capillaries. Resistance arteries and arterioles primarily function to control systemic blood pressure. The resistance arteries are composed of a layer of endothelial cells oriented parallel to the direction of blood flow, which are separated by a matrix layer termed the internal elastic lamina from several layers of smooth muscle cells oriented perpendicular to the direction of blood flow. Cells within the vessel walls communicate in a homocellular and heterocellular fashion to govern luminal diameter, arterial resistance, and blood pressure. At rest, potassium currents govern the basal state of endothelial and smooth muscle cells. Multiple stimuli can elicit rises in intracellular calcium levels in either endothelial cells or smooth muscle cells, sourced from intracellular stores such as the endoplasmic reticulum or the extracellular space. In general, activation of endothelial cells results in the production of a vasodilatory signal, usually in the form of nitric oxide or endothelial-derived hyperpolarization. Conversely, activation of smooth muscle cells results in a vasoconstriction response through smooth muscle cell contraction. © 2022 American Physiological Society. Compr Physiol 12: 1-35, 2022.

Mineral requirements for mitochondrial function: A connection to redox balance and cellular differentiation

Professor Bruce Ames demonstrated that nutritional recommendations should be adjusted in order to 'tune-up' metabolism and reduce mitochondria decay, a hallmark of aging and many disease processes. A major subset of tunable nutrients are the minerals, which despite being integral to every aspect of metabolism are often deficient in the typical Western diet. Mitochondria are particularly rich in minerals, where they function as essential cofactors for mitochondrial physiology and overall cellular health. Yet substantial knowledge gaps remain in our understanding of the form and function of these minerals needed for metabolic harmony. Some of the minerals have known activities in the mitochondria but with incomplete regulatory detail, whereas other minerals have no established mitochondrial function at all. A comprehensive metallome of the mitochondria is needed to fully understand the patterns and relationships of minerals within metabolic processes and cellular development. This brief overview serves to highlight the current progress towards understanding mineral homeostasis in the mitochondria and to encourage more research activity in key areas. Future work may likely reveal that adjusting the amounts of specific nutritional minerals has longevity benefits for human health.

Electron Microscopic Observations on Smooth Muscles of Cat Nicitating Membrane

The cat nictitating membrane is commonly used for physiological and pharmacological study, since its smooth muscles are virtually all innervated by sympathetic nerves. Thus, morphological studies at the fine structural level have been focussed to the innervation of the nictitating membrane, but not to the smooth muscle proper.

The nictitating membranes of adult cats were fixed in situ in protruded state with 2% formaldehyde and 2% glutaraldehyde in 0.1 M cacodylate buffer (pH. 7.3). The tissues were then dissected out to continue fixation in the same buffer. After rinse in 10% sucrose in the same buffer, the tissue blocks were postfixed in 1% OsO4 and embedded in Epon.

Role of Mitochondria in Cerebral Vascular Function: Energy Production, Cellular Protection, and Regulation of Vascular Tone

Mitochondria not only produce energy in the form of ATP to support the activities of cells comprising the neurovascular unit, but mitochondrial events, such as depolarization and/or ROS release, also initiate signaling events which protect the endothelium and neurons against lethal stresses via pre-/postconditioning as well as promote changes in cerebral vascular tone. Mitochondrial depolarization in vascular smooth muscle (VSM), via pharmacological activation of the ATP-dependent potassium channels on the inner mitochondrial membrane (mitoKATP channels), leads to vasorelaxation through generation of calcium sparks by the sarcoplasmic reticulum and subsequent downstream signaling mechanisms. Increased release of ROS by mitochondria has similar effects. Relaxation of VSM can also be indirectly achieved via actions of nitric oxide (NO) and other vasoactive agents produced by endothelium, perivascular and parenchymal nerves, and astroglia following mitochondrial activation. Additionally, NO production following mitochondrial activation is involved in neuronal preconditioning. Cerebral arteries from female rats have greater mitochondrial mass and respiration and enhanced cerebral arterial dilation to mitochondrial activators. Preexisting chronic conditions such as insulin resistance and/or diabetes impair mitoKATP channel relaxation of cerebral arteries and preconditioning. Surprisingly, mitoKATP channel function after transient ischemia appears to be retained in the endothelium of large cerebral arteries despite generalized cerebral vascular dysfunction. Thus, mitochondrial mechanisms may represent the elusive signaling link between metabolic rate and blood flow as well as mediators of vascular change according to physiological status. Mitochondrial mechanisms are an important, but underutilized target for improving vascular function and decreasing brain injury in stroke patients. © 2016 American Physiological Society. Compr Physiol 6:1529-1548, 2016.

Mitochondrial mechanisms in cerebral vascular control: shared signaling pathways with preconditioning

Mitochondrial-initiated events protect the neurovascular unit against lethal stress via a process called preconditioning, which independently promotes changes in cerebrovascular tone through shared signaling pathways. Activation of adenosine triphosphate (ATP)-dependent potassium channels on the inner mitochondrial membrane (mitoKATP channels) is a specific and dependable way to induce protection of neurons, astroglia, and cerebral vascular endothelium. Through the opening of mitoKATP channels, mitochondrial depolarization leads to activation of protein kinases and transient increases in cytosolic calcium (Ca(2+)) levels that activate terminal mechanisms that protect the neurovascular unit against lethal stress. The release of reactive oxygen species from mitochondria has similar protective effects. Signaling elements of the preconditioning pathways also are involved in the regulation of vascular tone. Activation of mitoKATP channels in cerebral arteries causes vasodilation, with cell-specific contributions from the endothelium, vascular smooth muscles, and nerves. Preexisting chronic conditions, such as insulin resistance and/or diabetes, prevent preconditioning and impair relaxation to mitochondrial-centered responses in cerebral arteries. Surprisingly, mitochondrial activation after anoxic or ischemic stress appears to protect cerebral vascular endothelium and promotes the restoration of blood flow; therefore, mitochondria may represent an important, but underutilized target in attenuating vascular dysfunction and brain injury in stroke patients.

Role of calcium-activated potassium channels in the genesis of 3,4-diaminopyridine-induced periodic contractions in isolated canine coronary artery smooth muscles

We found that 3,4-diaminopyridine (3,4-DAP), a voltage-gated potassium channel (K(V)) inhibitor, elicits pH-sensitive periodic contractions (PCs) of coronary smooth muscles. Underlying mechanisms of PCs, however, remained to be elucidated. The present study was performed to examine the roles of ion channels in the genesis of PCs. To determine the electromechanical changes of smooth muscles, isolated coronary arterial rings from beagles were suspended in organ chambers filled with Krebs-Henseleit solution, and 10(-2) M 3,4-DAP was added to elicit PCs. 3,4-DAP caused periodic spike-and-plateau depolarization accompanied by contraction. PCs were not produced when the CaCl(2) concentration in the chamber was ≤ 0.3 × 10(-3) or ≥ 10(-2) M. PCs were eliminated by a CaCl(2) concentration ≥ 5 × 10(-3) M or by lowering pH below 7.20 with HCl and recovered by the addition of iberiotoxin or charybdotoxin, which inhibit large-conductance calcium-activated potassium channels (K(Ca)), or by elevating pH above 7.35 with NaOH. PCs, as well as the spike-and-plateau depolarization, were eliminated by nifedipine, which inhibits L-type voltage-gated calcium channels (Ca(V)). Influx of Ca(2+) through L-type Ca(V), which was opened because closing of K(Ca), secondary to 3,4-DAP-induced closing of K(V), resulted in contraction; the intracellular Ca(2+) increased by this influx opened K(Ca), leading to closure of Ca(V) and consequent cessation of Ca(2+) influx with resultant relaxation. These processes were repeated spontaneously to cause PCs. H(+) and OH(-) were considered to act as the opener and closer of K(Ca), respectively.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

A review of recent insights into the role of the sarcoplasmic reticulum and Ca entry in uterine smooth muscle

The uterine sacroplasmic reticulum (SR) takes up and stores calcium [Ca], using an ATPase (SERCA) and the Ca-buffering proteins, calsequestrin and calreticulin. This stored Ca can be released via IP(3)-gated Ca channels. Decreases in luminal Ca concentration [Ca] have been directly measured following agonist stimulation. During spontaneous contractions however, there appears to be no involvement of the SR, as Ca entry and efflux across the plasma membrane account for these phasic contractions. After over-viewing current knowledge concerning SR structure and function, we highlight three areas of research which suggest new ways of looking at the role of the SR in the uterus, although they may be controversial or speculative at the moment. Firstly, we review the evidence for the function, if any, of Ca-induced SR Ca release channels, the ryanodine receptor (RyR) and the lack of Ca sparks (the elemental release events from RyRs), in the uterus. Secondly, we ask does regulation of SERCA by the accessory protein, phospholamban, occur in the uterus and what is the effect of knocking out phospholamban on uterine activity? Thirdly, we address the question of when and how store-operated Ca entry occurs in the myometrium. By analogy with other, usually less excitable tissues, is there a mechanism that links store Ca depletion to plasma membrane Ca entry in smooth muscle cells within intact uterus and is it physiologically relevant and regulated? Are the recently described proteins ORAI and STIM-1 involved in uterine store-operated Ca entry? We end the review by integrating these new insights with previous data to present a new working model of the SR in the uterus.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

Dose-dependent effects of strontium on osteoblast function and mineralization

BACKGROUND

Strontium-ranelate is now being used in the treatment of osteoporosis in elderly patients. As the majority of these patients already have a decreased renal function they are at an increased risk for accumulation of the element. Recent findings from epidemiologic studies in dialysis patients and experimental data obtained in a chronic renal failure (CRF) rat model established a dose-related multiphasic effect of strontium (Sr) on bone formation. To confirm these in vivo findings an in vitro set-up, consisting of primary rat osteoblast cultures, was applied. Sr was added to the culture medium at concentrations of 0, 0.5, 1.0, 2.0, 5.0, 20, and 100 microg/mL, respectively.

METHODS

Calcium incorporation (index of mineralization) and alkaline phosphatase activity were determined in the medium during the culture period, while at the end of the experiment, nodule formation (mineralized + unmineralized area) was quantified using a digital imaging system. mRNA synthesis of various osteoblast specific genes was assessed by means of reverse transcription polymerase chain reaction (RT-PCR).

RESULTS

Compared to the control group (0 microg/mL Sr), a significantly reduced nodule formation in the presence of an intact mineralization was found for the lowest 0.5 and 1 microg/mL Sr doses, suggesting an impaired in vitro osteoblast differentiation. Both nodule formation and mineralization were normal for the 2 and 5 microg/mL doses. For the highest Sr doses (20 and 100 microg/mL) a reduced mineralization was observed in the presence of an intact nodule formation indicating an inhibitory effect on the hydroxyapatite formation. The alkaline phosphatase activity reflected the multiphasic pattern of the nodule formation while the calcium incorporation corresponded with the pattern of nodular mineralization. No variations in cell proliferation were found. RT-PCR revealed that Sr interfered with the osteoblast at the level of the mRNA synthesis of several relevant genes.

CONCLUSION

Using the proposed in vitro model we confirmed the multiphasic effect of Sr on bone formation previously demonstrated in a CRF rat model. The data presented allow us to suggest that at low concentrations Sr interferes with the bone formation at the level of cell differentiation, whereas at high concentrations the disturbed mineralization in the presence of an intact nodule formation is indicative for a physicochemical interference of Sr with the hydroxyapatite formation.

Sustained norepinephrine contraction in the rat portal vein is lost when Ca(2+) is replaced with Sr(2+)

Agonist-induced activation of smooth muscle involves a rise in intracellular Ca(2+) concentration and sensitization of myosin light chain phosphorylation to Ca(2+). Sr(2+) can enter through Ca(2+) channels, be sequestered and released from sarcoplasmic reticulum, and replace Ca(2+) in activation of myosin light chain phosphorylation. Sr(2+) cannot replace Ca(2+) in facilitation of agonist-activated Ca(2+)-dependent nonselective cation channels. It is not known whether Sr(2+) can replace Ca(2+) in small G protein-mediated sensitization of phosphorylation. To explore mechanisms involved in alpha-receptor-activated contractions in smooth muscle, effects of replacing Ca(2+) with Sr(2+) were examined in rat portal vein. Norepinephrine (NE) at >3.0 x 10(-7) M in the presence of Ca(2+) resulted in a strong sustained contraction, whereas this sustained component was absent in the presence of Sr(2+); only the amplitude of phasic contractions increased. Pretreatment with low (approximately 0.05 mM) free Ca(2+) followed by 2.5 mM Sr(2+) resulted in a sustained component of the NE response. In beta-escin-permeabilized preparations, phenylephrine in the presence of GTP or guanosine 5'-O-(3-thiotriphosphate) alone induced sensitization to Sr(2+). In conclusion, a Ca(2+)-regulated membrane/channel process is required for the sustained component of NE responses in rat portal vein. Sensitization alone is not responsible for the sustained phase of the NE contraction.

Role of sarcoplasmic reticulum in regulation of tonic contraction of rabbit basilar artery

Superficial sarcoplasmic reticulum (SR) regulates smooth muscle force development directly by Ca(2+) release and removal to and from the cytoplasm (Somlyo and Somlyo. J Cardiovasc Pharmacol 8, Suppl 8: S42-S47, 1986) by buffering Ca(2+) influx and contributing to Ca(2+) extrusion (Mueller and van Breemen. Nature 281: 682-683, 1979) and indirectly by releasing Ca(2+) near Ca(2+)-activated K(+) channels (K(Ca)) to hyperpolarize the plasma membrane (Bolton and Imaizumi. Cell Calcium 20: 141-152, 1996 and Nelson et al. Science 270: 633-637, 1995). In the rabbit basilar artery, relative contributions of direct effects and those mediated through activation of K(Ca) were evaluated by measuring force and intracellular Ca(2+) concentration ([Ca(2+)](i)) in response to the SR-depleting agents thapsigargin and ryanodine and the large conductance K(Ca) (BK(Ca)) blockers iberiotoxin (IbTX) and tetraethylammonium ion (TEA). A large contraction was observed in response to K(Ca) blockade with either 3 mM TEA or 100 nM IbTX and also after addition of 10 microM ryanodine or 2 microM thapsigargin. When K(Ca) was blocked first with TEA or IbTX, subsequent addition of thapsigargin or ryanodine also increased force. Measurements of fura 2 fluorescence showed parallel increases in [Ca(2+)](i) in response to sequential blockade of sarco(endo)plasmic reticulum Ca(2+)-ATPase and K(Ca) regardless of the order of application. It appears that a significant fraction of K(Ca) remains activated in the absence of SR function and that SR contributes to relaxation after blockade of K(Ca). We found that depletion of SR before stimulating Ca(2+) influx through voltage-gated Ca(2+) channels markedly reduced force development rate and that thapsigargin abolished this effect. We conclude that the SR of rabbit cerebral arteries modulates constriction by direct and indirect mechanisms.

Activation of tyrosine kinase pathway by vanadate in gallbladder smooth muscle

Vanadate, an inhibitor of tyrosine phosphatase activity, might induce gallbladder contraction through the stimulation of the tyrosine kinase pathway. The aim of this study was to characterize the effects of vanadate in the guinea pig gallbladder smooth muscle. Vanadate exerts contractile effects which are not mediated by neurotransmitter release. The tyrosine kinase inhibitor genistein nearly abolished vanadate contraction, suggesting that an increase in protein tyrosine phosphorylation mediates the actions of vanadate. This suggestion was confirmed by Western blot analysis. Vanadate contractions were reduced in the presence of methoxyverapamil or in Ca(2+)-free medium, suggesting that vanadate may induce Ca(2+) influx. Neither inactivation of the Na(+)/K(+) pump nor reversal of the Na(+)/Ca(2+) exchanger can account for vanadate's actions. Vanadate contractile effects were reduced by indomethacin, as well as mepacrine, the inhibitor of phospholipase A(2), but were not affected by phospholipase C inhibitors. Neither inhibitors of diacylglycerol lipase nor protein kinase C reduced the response induced by vanadate. These data indicate that the effects of vanadate on smooth muscle are mainly mediated by protein tyrosine phosphorylation and reveal a new link between tyrosine phosphorylation and arachidonic acid metabolism in the control of gallbladder smooth muscle contraction.

Further evidence for the heterogeneity of functional muscarinic receptors in guinea pig gallbladder

Previous studies have suggested the presence of multiple muscarinic receptor subtypes in guinea pig gallbladder smooth muscle, although the relative abundance and functional role of these subtypes remains an area of significant research efforts. The present study utilized both radioligand kinetic and functional experiments to further probe the nature of the muscarinic receptors in gallbladder smooth muscle and their mode of coupling to intra- and extra-cellular Ca(2+) sources. Dissociation kinetic studies using [3H]N-methylscopolamine ([3H]NMS) indicated that the binding profile in guinea pig gallbladder smooth muscle could not be reconciled with that expected for a single muscarinic receptor subtype, the latter determined in parallel experiments conducted on the cloned muscarinic M(1)-M(5) subtypes in Chinese hamster ovary (CHO) cells. Furthermore, comparison of the gallbladder data with the dissociation characteristics of [3H]NMS in guinea pig urinary bladder revealed a significantly different kinetic profile, with the urinary bladder, but not the gallbladder, demonstrating biphasic radioligand dissociation kinetics. In functional experiments, carbachol caused a concentration-dependent contraction of guinea pig gallbladder smooth muscle strips in Ca(2+)-free or 5 mM Sr(2+)-substituted physiological salt solutions (PSS) with amplitudes of the maximal contractions corresponding to 45.8+/-8.0% and 33.2+/-6.6% of control responses in normal PSS, respectively. Furthermore, the stimulus-response characteristics of carbachol-mediated contraction appeared significantly altered in Ca(2+)-free PSS relative to normal or Sr(2+)-substituted PSS. The antagonist, methoctramine (1x10(-7)-3x10(-5) M), exerted only a slight inhibition of carbachol (10(-5) M)-induced contractions in 5 mM Sr(2+)-substituted medium, whereas it was significantly more potent in antagonizing gallbladder contractions in response to 10(-5) M carbachol in the absence of extracellular Ca(2+). Both atropine and tripitramine were equipotent in antagonizing carbachol-induced contractions in Ca(2+)-free (pIC(50): 6.85+/-0.11 for atropine and 5.75+/-0.32 for tripitramine) and Sr(2+)-substituted media (pIC(50): 6.88+/-0.25 for atropine and 5.70+/-0.16 for tripitramine), and pirenzepine was only slightly more potent in Ca(2+)-free PSS (pIC(50): 5.66+/-0.23) than in Sr(2+)-substituted PSS (pIC(50): 5.33+/-0.21). Taken together, our data indicate that carbachol contracts guinea pig gallbladder by stimulating two distinct muscarinic receptor subtypes linked to extracellular Ca(2+) influx and intracellular Ca(2+) release. These two subtypes may represent the muscarinic M(3) and M(4) receptors, although the presence of the muscarinic M(2) receptor subtype is also suggested from the binding data.

Pharmacomechanical coupling: the role of calcium, G-proteins, kinases and phosphatases

The concept of pharmacomechanical coupling, introduced 30 years ago to account for physiological mechanisms that can regulate contraction of smooth muscle independently of the membrane potential, has since been transformed from a definition into what we now recognize as a complex of well-defined, molecular mechanisms. The release of Ca2+ from the SR by a chemical messenger, InsP3, is well known to be initiated not by depolarization, but by agonist-receptor interaction. Furthermore, this G-protein-coupled phosphatidylinositol cascade, one of many processes covered by the umbrella of pharmacomechanical coupling, is part of complex and general signal transduction mechanisms also operating in many non-muscle cells of diverse organisms. It is also clear that, although the major contractile regulatory mechanism of smooth muscle, phosphorylation/dephosphorylation of MLC20, is [Ca2+]-dependent, the activity of both the kinase and the phosphatase can also be modulated independently of [Ca2+]i. Sensitization to Ca2+ is attributed to inhibition of SMPP-1M, a process most likely dominated by activation of the monomeric GTP-binding protein RhoA that, in turn, activates Rho-kinase that phosphorylates the regulatory subunit of SMPP-1M and inhibits its myosin phosphatase activity. It is likely that the tonic phase of contraction activated by a variety of excitatory agonists is, at least in part, mediated by this Ca(2+)-sensitizing mechanism. Desensitization to Ca2+ can occur either through inhibitory phosphorylation of MLCK by other kinases or autophosphorylation and by activation of SMPP-1M by cyclic nucleotide-activated kinases, probably involving phosphorylation of a phosphatase activator. Based on our current understanding of the complexity of the many cross-talking signal transduction mechanisms that operate in cells, it is likely that, in the future, our current concepts will be refined, additional mechanisms of pharmacomechanical coupling will be recognized, and those contributing to the pathologenesis diseases, such as hypertension and asthma, will be identified.

Mechanisms of gallbladder hypomotility in pregnant guinea pigs

BACKGROUND & AIMS

Gallbladder muscle contraction becomes impaired during pregnancy. This study was designed to investigate the mechanisms of gallbladder hypomotility induced by pregnancy in guinea pigs.

METHODS

Gallbladder muscle cells were obtained by enzymatic digestion. Cell contraction was expressed as percent shortening of initial control cell length.

RESULTS

Contraction induced by cholecystokinin (CCK)-8 or guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) was reduced in muscle cells from pregnant guinea pigs. The response to KCl or D-myo-inositol 1,4, 5-trisphosphate was not different between controls and pregnant animals. These findings suggest that impaired contraction in pregnancy might be caused by defective G protein activation. The function and content of G proteins were examined by using [35S]GTPgammaS binding and G protein subunit quantitation. In female controls, CCK-8 at 1 micromol/L caused increased [35S]GTPgammaS binding to Galphai3 but not to Galphaq/11, Galphai1-2, or Galphas. GTPgammaS binding to Galphai3 induced by CCK-8 was reduced in gallbladder muscle from pregnant guinea pigs. Measurements of basal G proteins showed that the content of Galphai3 was significantly lower and the Galphas content was higher in muscles from pregnant guinea pigs than in controls.

CONCLUSIONS

Pregnancy may cause down-regulation of contractile G proteins such as Galphai3 and up-regulation of Galphas that mediates relaxation, resulting in impaired gallbladder muscle contraction.

Signal transduction pathways mediating CCK-induced gallbladder muscle contraction

The signal transduction that mediates CCK-induced contraction of gallbladder muscle was investigated in the cat. Contraction was measured by scanning micrometry in single muscle cells isolated enzymatically with collagenase. Production of D-myo-inositol 1,4, 5-trisphosphate (IP3) and sn-1,2-diacylglycerol (DAG) was quantitated using HPLC and TLC, respectively. Protein kinase C (PKC) activity was determined by measuring the phosphorylation of a specific substrate peptide from myelin basic protein, Ac-MBP-(4-14). CCK-induced contraction was blocked by incubation in strontium medium, pertussis toxin (PTx), and antibodies against Gialpha3 or betagamma-subunits but was not blocked by Ca2+-free medium or by antibodies against Gq/11alpha, Gialpha1-2, or Goalpha. The contraction induced by CCK was inhibited by the phospholipase C (PLC) inhibitor U-73122, anti-PLC-beta3 antibody, and the IP3 receptor antagonist heparin but was not inhibited by the the phospholipase D inhibitor propranolol or antibodies against PLC-beta1 or PLC-beta2. Western blot analysis of gallbladder muscle revealed the presence of PLC-beta2 and PLC-beta3 but not PLC-beta1. CCK caused a 94% increase in IP3 generation and an 86% increase in DAG generation. A low dose of CCK caused PKC translocation, and CCK-induced contraction was blocked by the PKC inhibitor H-7. A high dose of CCK, however, caused no PKC translocation, and its contraction was blocked by the calmodulin antagonist CGS9343B. In conclusion, CCK contracts cat gallbladder muscle by stimulating PTx-sensitive Gi 3 protein coupled with PLC-beta3, producing IP3 and DAG. Low doses activate PKC, whereas high doses activate calmodulin.

Impaired G protein function in gallbladder muscle from progesterone-treated guinea pigs

This study was designed to elucidate the mechanism of action of progesterone on gallbladder smooth muscle in guinea pigs. Adult male guinea pigs were treated with either progesterone (2 mg.kg-1.day-1) or saline for 7 days. Gallbladder muscle cells were isolated by enzymatic digestion with collagenase. Contractile responses to agonists were expressed as percent shortening from control cell length. [35S]guanosine 5'-O-(3-thiotriphosphate) ([35S]GTP gamma S)-binding properties of G proteins were assessed in crude membranes of gallbladder muscle with or without cholecystokinin octapeptide (CCK-8) stimulation. Gallbladder muscle cells from progesterone-treated guinea pigs exhibited an impaired contractile response to CCK-8, GTP gamma S, or aluminum fluoride but a normal response to potassium chloride or D-myo-inositol 1,4,5-trisphosphate compared with controls. Western blot analysis of gallbladder muscle revealed the presence of Gi1-2, Gi3, Gq/11, and Gs proteins. The maximal contraction induced by CCK-8 was blocked by pertussis toxin and Gi alpha 3-specific antibodies, but not by Gi alpha 1-2 or Gq/11 alpha antibodies. CCK-8 caused a significant increase in [35S]GTP gamma S binding to Gi alpha 3, but not to Gq/11 alpha or Gi alpha 1-2. The stimulation of Gi alpha 3 binding, however, was significantly reduced in gallbladder muscle membranes from progesterone-treated guinea pigs compared with that in control animals. In conclusion, progesterone might cause gallbladder hypomotility by downregulating Gi3 proteins.

Two calcium-binding sites mediate the interconversion of liver inositol 1,4,5-trisphosphate receptors between three conformational states

Cytosolic Ca2+ biphasically regulates Ins(1,4,5)P3-stimulated Ca2+ mobilization in liver [Marshall and Taylor (1993) J. Biol. Chem. 268, 13214-13220]. We have investigated the mechanisms underlying this biphasic control of Ca2+ mobilization in permeabilized hepatocytes by comparing the effects of Sr2+, Ba2+ and Ca2+ on the liver Ins(1,4,5)P3 receptor. Both Ca2+ and Sr2+ increased the binding of [3H]Ins(1,4,5)P3 to liver membranes by converting receptors from a low-affinity (KD approximately 35 nM) to a high-affinity (KD approximately 5 nM) state. Ba2+ (< or = 20 microM) did not affect [3H]Ins(1,4,5)P3 binding. At concentrations similar to those that caused an enhancement of [3H]Ins(1,4,5)P3 binding, Sr2+ (EC50 = 570 nM) and Ca2+ (EC50 = 200 nM) increased the sensitivity of the intracellular Ca2+ stores to Ins(1,4,5)P3. Further modest elevations in [Ca2+] (EC50 = 1.5 microM) inhibited Ins(1,4,5)P3-stimulated Ca2+ mobilization, whereas Sr2+ caused inhibition only when its concentration was very substantially increased (EC50 approximately 900 microM). Sr2+ is therefore only 3-fold less potent than Ca2+ in causing sensitization of Ins(1,4,5)P3-stimulated Ca2+ release, but 600-fold less potent in causing inhibition. Ba2+ neither sensitized ([Ba2+] < or = 20 microM) nor inhibited ([Ba2+] < or = 1 mM) Ins(1,4,5)P3-stimulated Ca2+ release, and did not inhibit either the sensitization of Ca2+ release evoked by Sr2+ or the inhibition of Ca2+ release evoked by Ca2+. Our results suggest that two distinct Ca(2+)-binding sites, which differ in their selectivities for bivalent cations, mediate the interconversion of Ins(1,4,5)P3 receptors between at least three different conformational states. These two Ca(2+)-binding sites, which may reside either on the Ins(1,4,5)P3 receptor itself or on distinct regulatory proteins, can be distinguished by their different selectivities for bivalent cations.

Cholecystokinin-coupled intracellular signaling in human gallbladder muscle

BACKGROUND/AIMS

It has been shown that cholecystokinin (CCK) contracts the gallbladder muscle by utilizing intracellular calcium, but the intracellular pathways have not been elucidated. The present study was designed to characterize the signal transduction pathways that mediate CCK-induced contraction of human gallbladder muscle.

METHODS

Single muscle cells were isolated from human gallbladders by enzymatic digestion with collagenase. Permeable cells were obtained by incubation with saponin. Protein kinase C (PKC) activity was determined by measuring the phosphorylation of a specific substrate peptide from myelin basic protein, Ac-MBP(4-14).

RESULTS

The inositol-1,4,5-trisphosphate (IP3) antagonist heparin blocked the contractions induced by CCK. The PKC inhibitor H-7 blocked the contractions caused by low, but not high, concentrations of CCK and IP3. In contrast, the calmodulin inhibitor CGS9343B blocked the contractions induced by high, but not low, doses of CCK and IP3. Furthermore, exogenously activated calmodulin blocked the PKC-mediated contraction induced by diacylglycerol. Direct measurements of PKC activity showed that low, but not high, CCK concentrations caused PKC translocation.

CONCLUSIONS

CCK contracts the gallbladder muscle via IP3-mediated calcium release. CCK activates the PKC pathway at low concentrations, whereas it activates the calmodulin pathway at high concentrations, which in turn inhibits the activation of PKC.

Calcium signalling in platelets and other nonexcitable cells

By virtue of their biological simplicity and widespread availability, platelets frequently have been used as a model system to study signal transduction. Such studies have revealed that changes in intracellular free calcium concentration are central to platelet functioning. The following article reviews current concepts of platelet structure and function, with particular emphasis on the mechanisms involved in platelet Ca2+ signalling.

Ultrastructural localization of calcium in the chick yolk sac membrane endodermal cells as revealed by cytochemistry and X-ray microanalysis

The yolk sac membrane (YSM) of the chick embryo transports calcium from the yolk into the embryonic circulation during the first half of development, but the intracellular pathway of calcium transport is poorly understood. In the present study, the ultrastructural localization of calcium was investigated in cells of the YSM of 9-day chick embryos. X-ray microanalysis as well as cytochemical techniques performed on yolk sac membrane cells treated with potassium oxalate, potassium ferricyanide and potassium antimonate demonstrated accumulation of calcium in yolk granules, digested yolk products, electron-dense bodies (EDBs; 100-400 nm diameter) and electron-dense granules (EDGs; 30-50 nm diameter). When strontium ions were injected into the yolk, they were incorporated into the endodermal cells and sequestered specifically in EDGs. From these results, we propose that calcium enters the endodermal cells by endocytosis of calcium-containing yolk granules, as well as through calcium channels in the apical cell membrane. In the cytoplasm, digested yolk products, EDBs, and EDGs act as sites of sequestration and accumulation of calcium. Extrusion of intracellular calcium into the extracellular space and embryonic circulation is accomplished by exocytosis of calcium-containing material and via an ion pump in the basal cell membrane.

Regulation of magnesium uptake and release in the heart and in isolated ventricular myocytes

Perfused rat hearts release or accumulate approximately 10% of total Mg2+ content when stimulated with norepinephrine (NE) or carbachol, respectively. Collagenase-dispersed rat ventricular myocytes increase or decrease total cell Mg2+ by 1 mM within 5 minutes when stimulated with these same transmitters. Measurements of Mg2+ transport using 28Mg or atomic absorbance spectrophotometry indicate that the rate and the extent of both stimulated Mg2+ efflux and influx are independent of the concentration of extracellular Mg2+ (0 to 1.2 mM). Mg2+ release induced by NE is rapidly reversed by the addition of carbachol, and Mg2+ uptake induced by carbachol is reversed by NE. Decreasing extracellular Na+ or Ca2+ decreases or abolishes Mg2+ efflux from myocytes. Cd2+ or other Ca2+ channel blockers also inhibit Mg2+ efflux in the presence of a physiological concentration of extracellular Ca2+. Replacement of extracellular Ca2+ with Sr2+ or with Mn2+ decreases or abolishes both stimulated efflux and influx of Mg2+. Redistribution of 85Sr in myocytes and in the supernatant indicates that under those conditions Sr2+ is released or accumulated by NE or carbachol in a manner similar to that of Mg2+. Hence, at least in the case of Sr2+, the inhibition of Mg2+ fluxes can be explained by the transport of Sr2+ rather than Mg2+ through the transport(s) systems. By contrast, replacement of extracellular Ca2+ with Ba2+ inhibits stimulated Mg2+ uptake but not Mg2+ release. These results indicate that cardiac myocytes have a major pool of Mg2+ that can be rapidly mobilized upon hormonal stimulation. The net uptake and release of Mg2+ are quantitatively similar and appear to be independent of the extracellular Mg2+ concentrations but are affected, to various degrees, by the presence of other cellular or extracellular cations.

Anti-ryanodine receptor antibody binding sites in vascular and endocardial endothelium

The ryanodine receptor (RyR) functions as the calcium release channel of the sarcoplasmic reticulum activated by electromechanical coupling in skeletal and cardiac muscles. In smooth muscle, inositol trisphosphate releases calcium from internal stores during pharmacomechanical coupling, but these cells also contain ryanodine-sensitive calcium stores. In this study, we establish the presence of anti-RyR antibody binding sites in vascular and endocardial endothelium. Both types of endothelia also contain messenger RNA, which hybridizes to a cardiac RyR isoform cDNA probe. Western blots of endothelial cell homogenates demonstrate the presence of a single, high molecular weight band of protein that corresponds to the cardiac RyR isoform. Confocal micrographs of endothelial cells labeled with a specific anti-RyR antibody reveal an intense fluorescent signal surrounding the nucleus and distributed in a nonhomogeneous pattern throughout the cytoplasm. This pattern of fluorescence is consistent with the electron microscopic distribution of the endoplasmic reticulum. The pattern of immunofluorescence seen with the anti-RyR antibody is distinctly different from that seen with the mitochondrial fluorophore rhodamine 123. Our findings suggest that the RyR plays a role in endothelial signaling.

Inositol trisphosphate restores impaired human gallbladder motility associated with cholesterol stones

BACKGROUND

Gallbladder motility is impaired in specimens with cholesterol stones but normal with pigment stones.

METHODS

Muscle cells obtained from 19 human gallbladders with cholesterol stones and 11 with pigment stones were enzymatically digested and contracted with cholecystokinin octapeptide (CCK-8), acetylcholine, and KCl.

RESULTS

Muscle cells from pigment stones had a greater contraction than cells from cholesterol stones. CCK-8-induced contraction was unaffected by calcium-free media but was blocked by strontium. Potassium-evoked contraction was blocked by a calcium-free media and unaffected by strontium. Inositol triphosphate (IP-3)-induced contraction was similar to the contraction caused by CCK-8 in permeable cells from pigment stones but was greater than the response to CCK-8 in cells from cholesterol stones.

CONCLUSIONS

Muscle cells from gallbladders with cholesterol stones contract less than cells from gallbladders with pigment stones; CCK-8-induced contraction only uses stored calcium; and IP-3 causes contractions of equal magnitude in cells from gallbladders with cholesterol and pigment stones. These abnormalities could result from an impaired receptor activation of the mechanism for IP-3 generation and release of stored calcium.

Ultrastructural localization of calcium in the chick chorioallantoic membrane as revealed by cytochemistry and X-ray microanalysis

The chorioallantoic membrane (CAM) of the chick embryo actively transports calcium from the egg shell into the embryonic circulation. To investigate the intracellular pathway of calcium transport across the CAM, ultrastructural localization of intracellular calcium in cells of the chorionic ectoderm (CE) was determined using cytochemical methods and X-ray microanalysis. Treatment of the CE with potassium oxalate, potassium ferricyanide or potassium pyroantimonate revealed large numbers of electron-dense granules (EDGs) in the ectodermal cells. These measure 30-40 nm in diameter, and are not membrane-bound. These granules were seen in all three cell types of the CE. The presence of calcium in the EDG was directly confirmed by X-ray microanalysis. When strontium or barium ions were applied to the shell membrane side of the CAM, the cells of the CE incorporated these divalent cations and sequestered them in granules (25-40 nm in diameter) in cytoplasm and mitochondria. This study indicates that calcium enters the CE cells by means other than endocytosis, as the EDGs are not membrane-bound, that all three types of the CE cells appear to function in transport of calcium from shell to embryo during embryogenesis, and that the EDG plays important roles in intracellular accumulation of calcium during the process of calcium transport across the chorioallantoic membrane.

Evaluation of platelet calcium ion mobilization by the use of various divalent ions

Divalent ion mobilization in human platelets was evaluated with Fura-2 fluorescence changes induced by Ca2+, Sr2+, Ba2+ and Mn2+. Extracellular Ca2+, Sr2+ and Ba2+ all entered thrombin-stimulated platelets. These divalent ions were also able to refill the intracellular Ca2+ storage sites which had been depleted of Ca2+ by ionomycin treatment, and were released from the storage sites upon thrombin stimulation. However, only the refilling of the storage sites with Ca2+ and Sr2+, but not with Ba2+, were capable of suppressing the opening state of Ca2+ channels assessed with Mn2+ influx. Efflux of intracellularly accumulated divalent ions was observed with Ca2+ and Sr2+ but not with Ba2+. These findings indicate that there are subtle differences in the Ca(2+)-binding domains of the various systems involved in Ca2+ mobilization in platelets, some of which discriminate Ba2+ while accepting Sr2+.

Sr2+ activates cross-bridge phosphorylation and latch state in smooth muscle

Sr2+ induced myosin phosphorylation and stress development in both skinned and K+-depolarized, Ca2+-depleted, intact swine carotid media. Although higher concentrations of Sr2+ than Ca2+ were required for phosphorylation and stress development, the dependence of stress on phosphorylation was the same in intact tissues. K+ depolarization in the presence of 5 mM Sr2+ produced a transient in phosphorylation (53.2 +/- 5.1% at 1 min, falling to a steady-state value of 21.7 +/- 2.0%) in Ca2+-depleted tissues in which intracellular stores were refilled with Sr2+. Stress developed without a transient (T1/2 = 0.70 min) to a steady state of 89.7 +/- 2.0% of the stress induced by K+ depolarization in the presence of 1.6 mM Ca2+ (K-PSS). Cross-bridge cycling rate as measured by isotonic shortening velocity was proportional to myosin phosphorylation throughout the contraction. When intracellular stores were not refilled with Sr2+, phosphorylation rose to a sustained value of 28.8 +/- 2.7% and stress developed slowly (T1/2 = 2.9 min) to a steady state of 95.9 +/- 1.5% K-PSS-induced stress. Therefore, an initial phosphorylation transient induced by intracellular Sr2+ release only accelerated stress development without significant effects on steady-state stress or phosphorylation (as was true for Ca2+- induced responses). We concluded that Sr2+ substitutes for Ca2+ in phosphorylation and regulation of the latch state in the swine carotid media.

Na+-Ca2+ exchange in cultured vascular smooth muscle cells

Vascular smooth muscle cells (VSMC) contract as intracellular free calcium ([Ca2+]i) rises. While Na+-Ca2+ exchange has been proposed to contribute to transmembrane Ca2+ flux, its role in cultured VSMC is unknown. Accordingly, we have investigated the role of Na+-Ca2+ exchange in unidirectional and net transmembrane Ca2+ fluxes in cultured rat aortic VSMC under basal conditions and following agonist-mediated stimulation. Transmembrane Ca2+ uptake was significantly increased in response to a low external Na+ concentration ([Na+]o) compared with 140 mM [Na+]o. Na+-dependent Ca2+ uptake in response to low [Na+]o was further increased by intracellular Na+ loading by preincubation of the VSMC with 1 mM ouabain. Under steady-state conditions, Ca2+ content varied inversely with [Na+]o, increasing from 1.0 nmol Ca2+/mg protein at 140 mM [Na+]o to 4.0 nmol Ca2+/mg protein at 20 mM [Na+]o. Increasing [K+]o to 55 mM also enhanced Na+-dependent Ca2+ influx. Augmentation of Ca2+ uptake with K+ depolarization was not significantly inhibited by the calcium channel antagonist verapamil. Transmembrane Ca2+ efflux was increased in response to 130 mM [Na+]o compared with zero [Na+]o (iso-osmotic substitution with choline+), and was further stimulated by the vasoconstrictor angiotensin II, which is known to elevate [Ca2+]i. These changes in [Ca2+]i were studied directly using fura-2 fluorescence measurements. Elevated [Ca2+]i levels returned to baseline more rapidly in the presence of normal (130 mM) [Na+]o compared with zero [Na+]o (iso-osmotic substitution with choline+). These findings suggest that a bidirectional Na+-Ca2+ exchange mechanism is present in cultured rat aortic VSMC.(ABSTRACT TRUNCATED AT 250 WORDS)

Frontiers in electron probe microanalysis: application to cell physiology

The application of electron probe microanalysis techniques, using X-ray and electron energy loss instruments, to problems in cell physiology is reviewed. The details of the special methodological requirements for the analysis of cryosections at high spatial resolution in an analytical electron microscope are discussed together with a comprehensive review of data obtained on major organ systems and cell types.

Fenoverine: a novel synchronizer of smooth muscle motility by interference with cellular calcium flow

Several in vivo and in vitro test models were used to study the mechanism of action of fenoverine, a novel synchronizer of smooth muscle motility. In vivo, the effects of fenoverine were tested in the rabbit proximal colon, recording its ability to modify the electromyographic activity, either spontaneous or electrically elicited, in the presence or absence of atropine. Fenoverine did not influence the spontaneous motility nor did it modify the effects of atropine, but it significantly reduced the contractions elicited by electrical stimulation. In vitro, isolated rabbit and rat colon and isolated rat myometrium were used. In rabbit colon, fenoverine failed to influence the spontaneous motility as well as the effects of atropine, while significantly inhibiting the electrically elicited excitatory junction potential. Fenoverine also significantly inhibited the isometric contractions induced in rat myometrium and colon by electrical stimulation, by depolarization with hyperpotassic solution, as well as those induced by acetylcholine in calcium-free/EGTA medium, with ID50 ranging from 8.10(-7) to 3.1.10(-6) M except in the isolated colon following K+ depolarization (5.10(-5) M), all actions compatible with a calcium-modulating mechanism. Based on the reported data, it is concluded that fenoverine does not act as a muscarinic or opiate-receptor antagonist, but that its main mechanism of action is due to modulation of the calcium gradient across the muscular cell membrane by regulating the influx of the extracellular calcium and/or the release of the intracellular pool.

Electron cytochemical study of the muscle cell surface

The lectins wheat germ agglutinin and limulus polyphemus were used as cytochemical probes to study the ultrastructural localization of sialic acid at the cell surface of rat muscle fibers. In addition cytochemical studies employing strontium as an electron-dense marker were also carried out to investigate cation binding sites at the muscle cell surface. The results showed binding of the lectins to the glycocalyx, caveolae and the basal lamina of the muscle fibers. These binding sites matched the ones observed in the cytochemical studies using strontium as a marker. Based on these observations we suggest that the glycocalyx, caveolae and the basal lamina of the muscle fiber may be involved in the binding of Ca++ and that significant amounts of Ca++ may be normally present at the muscle cell surface.

An electron microscopic study of cerebral vasospasm with resultant myonecrosis in cases of subarachnoid haemorrhage, meningitis and trans-sylvian surgery

Electron microscopic data on the development of myonecrosis following cerebral vasospasm associated with subarachnoid haemorrhage, meningitis and trans-sylvian surgery are presented. The basic feature of myonecrosis was dissolution of myofilaments with resultant fine granular or filamentous material. The disintegrating cytoplasm often contained numerous glycogen granules, dense bodies, autophagic vacuoles and myelin-like membranous bodies. A well-developed sarcoplasmic reticulum was preserved despite myofilament dissolution, while mitochondria showed marked swelling. The nuclei showed either dilution of chromatin or pyknotic change. The basal lamina was remarkably thickened and maintained an irregular outline of the necrotic smooth muscle cells. Enlarged intercellular space contained abundant cellular debris, vesicular structures and connective tissue fibres. The pathogenesis of these changes is discussed.

Calcium-sensitive cellular and subcellular transport of sodium, potassium, magnesium, and calcium in sodium-loaded vascular smooth muscle. Electron probe analysis

Electron probe x-ray microanalysis of the composition of rabbit portal anterior mesenteric vein smooth muscle was performed following sodium loading and washout into sodium-free lithium solutions. Sodium and lithium were also measured with atomic absorption spectrophotometry. Cellular uptake of sodium and loss of potassium during sodium loading were much faster at high (37 degrees C) than at low (2 degrees C) temperature, as was the passive ouabain-resistant uptake of potassium during lithium washout. The loss of sodium at 2 degrees C into lithium solution consisted of two components: a rapid efflux that was complete by 30 minutes, and a slow component that required at least 24 hours for completion. The amount of sodium lost through the first component (approximately 200-300 mmol/kg dry weight) was relatively independent of the amount of sodium loading. The loss of cellular sodium at 2 degrees C, after 30 minutes, was accompanied by a gain of cellular lithium. Ouabain-resistant sodium loss and lithium and potassium uptake were markedly accelerated at 37 degrees C; sodium loss was complete (1200 mmol sodium/kg dry weight lost) by 30 minutes of washout. Sodium-loaded cells also lost chloride ion and gained magnesium during sodium efflux at 37 degrees C. Mitochondrial and nuclear sodium and potassium were correlated with the respective cytoplasmic concentrations during both sodium loading and sodium washout, indicating the relatively rapid equilibration of the monovalent ions between the cytoplasm and organelles. Calcium-free solutions markedly inhibited the ouabain-resistant sodium and chloride ion effluxes and potassium influx in muscles incubated, after sodium loading, in lithium solutions at 37 degrees C. These fluxes could be restored to near normal values by 0.2 mM calcium. The calcium sensitivity of the ouabain-resistant sodium, potassium, and chloride ion fluxes observed in this and other studies raises the possibility that some abnormalities of monovalent ion transport observed in cells of hypertensives are secondary to changes in cellular calcium.

Ca2+ compartments in saponin-skinned cultured vascular smooth muscle cells

A method for saponin skinning of primary cultured rat aortic smooth muscle cells was established. The saponin-treated cells could be stained with trypan blue and incorporated more 45Ca2+ than the nontreated cells under the same conditions. At low free Ca2+ concentration, greater than 85% of 45Ca2+ uptake into the skinned cells was dependent on the extracellularly supplied MgATP. In the intact cells, both caffeine and norepinephrine increased 45Ca2+ efflux. In the skinned cells, caffeine increased 45Ca2+ efflux, whereas norepinephrine did not. The caffeine-releasable 45Ca2+ uptake fraction in the skinned cells appeared at 3 X 10(-7) M Ca2+, increased gradually with the increase in free Ca2+ concentration, and reached a plateau at 1 X 10(-5) M Ca2+. The 45Ca2+ uptake fraction, which was significantly suppressed by sodium azide, appeared at 1 X 10(-5) M Ca2+ and increased monotonically with increasing free Ca2+ concentration. The results suggest that the caffeine-sensitive Ca2+ store, presumably the sarcoplasmic reticulum, plays a physiological role by releasing Ca2+ in response to norepinephrine or caffeine and by buffering excessive Ca2+. The 45Ca2+ uptake by mitochondria appears too insensitive to be important under physiological conditions.

Calcium release by noradrenaline from central sarcoplasmic reticulum in rabbit main pulmonary artery smooth muscle

The subcellular composition of relaxed and noradrenaline-contracted rabbit main pulmonary artery smooth muscle cells was measured by electron probe X-ray microanalysis of cryosections of rapidly frozen tissue. Some of the preparations were made permeable with saponin and exposed to a known free Ca ion concentration, rapidly frozen, freeze-substituted, and also analysed by electron probe X-ray microanalysis. 98% of intracellular K could be replaced by Rb. This was done to remove the K peak that partially overlaps the Ca peak in the X-ray spectra. The final [Rb]i plus residual [K]i was not significantly different from the [K]i of normal tissue. The [Ca]i in Rb-containing tissue was not significantly different from the [Ca]i in normal, K-containing tissue. Non-mitochondrial micro-regions containing high [Ca] (up to 33 mmol/kg dry wt.) were found at sites 200 nm or more away from the plasma membrane. These micro-regions also contained high [P]. We consider the identification of these regions containing high [Ca] as sarcoplasmic reticulum (s.r.), validated by: (a) conventional electron micrographs that show no other structures in main pulmonary artery smooth muscle in sufficient quantity and location to account for the frequency of these regions, (b) the previous localization of strontium, a functional calcium analogue, in the central s.r. in these smooth muscles (Somlyo & Somlyo, 1971 a), (c) the present demonstration that the central s.r. in this tissue can accumulate large amounts of calcium oxalate. The proportion of regions containing high [Ca] (greater than 12.0 mmol/kg dry wt.) was significantly higher in relaxed (35 of 330 measurements) than in the contracted (14 of 337) tissues (P less than 0.005), or 26 of 34 vs. 6 of 31 high [Ca] measurements in regions identified as s.r. through their high phosphorus content (P less than 0.006). This difference is thought to represent Ca release from the central s.r. There was no significant difference (P greater than 0.05) between the distributions of P in relaxed and contracted smooth muscle. The total cell [Ca]i in relaxed Rb-containing tissue, measured with randomly positioned small probes (3.6 mmol/kg dry wt.), was the same as that measured with large defocused probes, indicating the validity of random sampling. A mathematical model was used to estimate the frequency of including s.r. (35 nm diameter and 5% of cell volume) by a randomly positioned electron probe (50 nm), because we could not visualize s.r. in the cryosections.(ABSTRACT TRUNCATED AT 400 WORDS)