A new method for measuring the kinetics of transmembrane calcium-45 efflux in the smooth muscle of the guinea pig tanea coli

Mechanisms for Ca signaling in vascular smooth muscle: resolved from 45Ca uptake and efflux experiments

Established cell lines are now widely used in experiments concerning vascular smooth muscle (VSM) function; however, considerable evidence suggests that cultured VSM cells are functionally different from VSM cells in intact blood vessels. In order to test the hypothesis that calcium signaling mechanisms are comparable in these two preparations, we developed a new method for high resolution 45Ca efflux studies in A7r5 cells. Briefly, this method involves plating cells in the lumen of a tubular glass efflux chamber and, after loading the cells with 45Ca, perfusing the chamber with a physiological saline solution and collecting the effluent. Using this method we found that the plasma membrane in cultured cells is not rate limiting for calcium efflux, since the efflux curves from both permeabilized and intact cells are kinetically the same. We also found the plasma membrane is not rate limiting in whole aortic segments by using a depolarizing solution followed by dihydropyridine solution. Thus, we demonstrated that the data obtained from cells or tissues with intact membranes reveal information about the intracellular stores (sarcoplasmic reticulum and mitochondria). Combining efflux data with a detailed kinetic model of cellular Ca transport allows least-squares estimation of the rate constants for release and uptake of Ca2+ by intracellular stores with a high degree of confidence (CV < 25%) as well as the Ca2+ contents and transmembrane fluxes associated with these stores. Quantitative comparison of results obtained from A7r5 cells with those we previously obtained for rabbit aortic segments reveals marked similarities and suggests that A7r5 cells serve as excellent model experiments for VSM cell Ca2+ homeostasis.

Effects of methoxamine and barium on 45Ca2+ fluxes in the rat vas deferens

The aim of this study was to determine whether methoxamine and barium stimulate 45Ca2+ uptake or efflux in the rat vas deferens in a manner that correlates with their contractile activity, and whether 45Ca2+ movements are inhibited by verapamil or nifedipine. Basal La(3+)-resistant (cellular) 45Ca2+ uptake was significantly greater in the epididymal half (791 +/- 27 nmol g-1) than in the prostatic half (654 +/- 14 nmol g-1) of the rat vas deferens and was unaffected by verapamil (61 microM) or nifedipine (14 microM). Methoxamine (8 microM) was without effect on 45Ca2+ uptake in either half but BaCl2 (1 mM) increased 45Ca2+ uptake by 31% in the prostatic half and by 22% in the epididymal half. The barium-induced increases in 45Ca2+ uptake were markedly reduced or abolished by verapamil (2 microM) or nifedipine (0.3 microM), which at these concentrations have no effect on the rhythmic contractions but abolish the initial small phasic contraction induced by barium. The basal rate of 45Ca2+ efflux from the intact vas deferens (into Ca2+ containing Krebs-Henseleit solution or into Ca-free Krebs-Henseleit solution +/- EGTA 0.05 mM) was not affected by verapamil (61 microM) or nifedipine (14 microM). Methoxamine (8 microM) produced a marked, transient and reversible increase in 45Ca2+ efflux into 2.5 mM CaCl2 Krebs-Henseleit in 50% of the intact vasa deferentia examined which was augmented by verapamil (61 microM). BaCl2 (1 mM) produced a small increase in 45Ca2+ efflux into Ca(2+)-containing and Ca(2+)-free Krebs-Henseleit solutions from some intact vasa deferentia and this was not inhibited by nifedipine (14 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Inward movement of Ca2+ in K(+)-depolarized rat vascular smooth muscle

The kinetics of the Ca2+ (45Ca2+) uptake in segments of rat aorta and its relationship with tension development in the presence of K+ concentrations ([K+]o) ranging from 3.5 to 100 mM was investigated. It was found that: 1) The curve relating 45Ca2+ uptake and time was virtually linear during the first 3 min. Therefore, the influx calculated as uptake in 3 min/3 min was not significantly different from that calculated as the slope of the uptake curve. The actual uptake of the cation during the loading period would be underestimated by about 30 percent if the loss of Ca2+ from that compartment during a 2 h clearing of the extracellular space in La3+ medium, is ignored. 3) 45Ca2+ activity remaining in the tissue after a 3 min labelling period followed by a 2 h washout can reliably be corrected to calculate Ca2+ influx. 4) The K(+)-induced tension development showed no significant correlation with the total exchangeable Ca2+ while it appeared as a steep function of Ca2+ influx between 15 and 80 mM [K+]o. 5) In [K+]o = 100 mM Ca2+ influx was 5 to 6 times larger than in normal [K+]o (5.3 mM). The estimated Ca2+ permeability (PCa) increased linearly as a function of [K+]o (5.3 to 100 mM). In [K+]o = 100 mM, PCa was estimated to be about 17-fold that in [K+]o = 5.3 mM. Similarly, the increased in Ca2+ conductance (GCa) for the same change in [K+]o was estimated to be 14-fold.

Polyamines mediate androgenic stimulation of calcium fluxes and membrane transport in rat heart myocytes

The androgenic steroid hormone testosterone induced an early (less than 30-60 seconds) stimulation of endocytosis, hexose transport, and amino acid transport, monitored by the temperature-sensitive uptake of horseradish peroxidase, 2-deoxyglucose, and alpha-aminoisobutyrate, respectively, in rat ventricle cubes and acutely isolated ventricular myocytes. This stimulation was time- and concentration-dependent and was maximal at 10(-9) to 10(-8) M testosterone, consistent with androgen-receptor mediation. EGTA (2.5 mM), La3+ (1 mM), and verapamil (100 microM) ablated the hormonal response. The calcium ionophore A23187 (10 microM) induced an acute stimulation of endocytosis, amino acid transport, and hexose transport which was not further increased by testosterone (10(-8) M), suggesting a common effector pathway. Testosterone (10(-8) M) also evoked a rapid (less than 30 seconds) stimulation of 45Ca influx and efflux. Testosterone (10(-8) M) induced a rapid (less than 5 seconds) transient increase in ornithine decarboxylase (ODC) activity peaking (twofold to threefold) at 60 seconds, and an early (15 seconds) transient accumulation of polyamines peaking at 60 seconds in isolated myocytes. The specific, irreversible ODC inhibitor alpha-difluoromethylornithine (DFMO, 5-10 mM) blocked the testosterone-evoked increase in ODC activity and polyamine levels and the stimulation of Ca2+ fluxes, endocytosis, hexose transport, and amino acid transport. Putrescine (0.5-1 mM), the ODC product, reversed DFMO inhibition and restored the increase in polyamines, 45Ca fluxes, and Ca2+-dependent membrane transport processes. These results demonstrate that rapid, transient ODC-regulated polyamine synthesis is essential for androgenic stimulation of Ca2+ fluxes and membrane transport processes in ventricular myocytes. These findings support a model for signal transduction in which newly synthesized polyamines serve as intracellular messengers to regulate transmembrane Ca2+ movements, Ca2+-dependent membrane transport functions, and other Ca2+- and polyamine-sensitive processes in cardiac myocytes.

Relaxing effects of adenosine in coronary artery in calcium-free medium

Adenosine relaxes the coronary arteries of various species through A2 receptors. The mechanism(s) by which adenosine causes relaxation of the coronary smooth muscle through changes in intracellular Ca levels is not understood. Therefore, the aim of this study was to evaluate the vasodilatory action of adenosine in normal and in Ca-free medium and the effect of adenosine on 45Ca efflux in Ca-free medium. Prostaglandin F2 alpha (10(-5) M) was used to induce tone in bovine coronary artery rings both in the presence and absence of Ca. Adenosine, 5'-N-ethyl-carboxamide adenosine (NECA) and N6-L-phenyl-isopropyl adenosine (L-PIA) produced concentration-dependent relaxation of the coronary artery rings when precontracted with prostaglandin F2 alpha. Both in normal and Ca-free medium, the order of potency for adenosine analogs (NECA greater than L-PIA greater than adenosine) was similar and 8-phenyltheophylline antagonized the relaxation response to adenosine and its analogs. In Ca-free medium, the concentration-response curves for adenosine and its analogs were shifted to the right, in a parallel fashion, from the curves obtained in Ca-containing solution. Removal of extracellular Ca reduced the maximum responses and slowed the rate of relaxation of adenosine and its analogs and the KB values for 8-phenyltheophylline were significantly increased. In Ca-free medium, adenosine was without an effect on 45Ca efflux in the presence of prostaglandin F2 alpha. The data suggest that the coronary vasodilatory action of adenosine has both extracellular Ca-dependent and -independent components.

Pharmacological characteristics of receptor-operated and potential-operated Ca2+ channels in rat aorta

In the rat aorta activation of the potential-operated Ca2+ channels by 100 mM K+ resulted in a greater 45Ca2+ influx than stimulation of the receptor-operated Ca2+ channels by norepinephrine (NE, 3 X 10(-7) M) or angiotensin II (AII, 10(-7) M). 45Ca2+ influx induced by NE was inhibited by prazosin (10(-7) M) but not by yohimbine (10(-6) M) while that by AII was abolished by [Sar1, Ile8]AII (10(-8) M). These receptor antagonists had no effect on the 45Ca2+ influx produced by K+. Bay k 8644 enhanced the influxes to low concentrations of NE and K+ while it was additive with the maximal concentration of NE but not with K+. 3-Isobutyl-1-methyl-xanthine and forskolin inhibited both the influx and efflux of 45Ca2+ elicited by NE but were ineffective against those caused by K+. Nifedipine blocked the efflux of 45Ca2+ induced by K+ but not that evoked by NE. However, both types of Ca2+ channel exhibited the same sensitivity to inhibition by Ca2+ entry blockers (nifedipine/verapamil) on 45Ca2+ influxes. These data suggest that in the rat aorta, the receptor-operated calcium channels and potential-operated calcium channels share similar structural characteristics. However, they are gated separately and distinctly by their respective activators.

Resolution of intracellular calcium metabolism in intact segments of rabbit aorta

A new method, based on computer-assisted kinetic analysis of 45Ca efflux data, was used to measure calcium contents and fluxes for extracellular and intracellular compartments in intact segments of rabbit aorta. After a 1-hour loading period, efflux data were collected for 8 hours using a flow-through tissue chamber. These long-term effluxes were necessary because information on intracellular calcium metabolism was concentrated in the slow components of the efflux curves while earlier components appeared to be dominated by washout of extracellular calcium. Intracellular compartments were identified as those whose calcium contents were altered by 10 microM phenylephrine. This method complements previous approaches by providing simultaneous estimates of compartmental calcium contents and fluxes without requiring the assumption of isotopic equilibrium and without recourse to standard wash techniques for removal of extracellular calcium. In normal, calcium-containing, bicarbonate-buffered physiological salt solution these compartments contained a total of approximately 300 nmol Ca/g wet aorta. Of this total, 55 nmol/g were associated with the slowest resolvable compartment whose turnover time was 170 minutes and whose exchange flux was 0.32 nmol min-1g-1. Two other intracellular compartments had turnover times of 30 minutes. One of these was phenylephrine releasable and contained 145 nmol/g; it exchanged calcium at 4.9 nmol min-1g-1. In normal physiological salt solution the plasma membrane was, surprisingly, not rate limiting for Ca efflux; and in 10 microM phenylephrine the membrane Ca flux was even greater, increasing 3.5-fold compared to control.

Kinetic identification of an intracellular calcium compartment sensitive to phosphate and dinitrophenol in intact isolated rabbit aorta

Previous work from this laboratory revealed the presence of at least three distinct intracellular calcium compartments in intact segments of rabbit aorta. In this study one of these intracellular compartments is shown to be sensitive to dinitrophenol and to increased extracellular phosphate. Intact aortic segments were loaded with 45Ca in bicarbonate-buffered physiologic salt solution for 1 hour, and then transferred to a flow-through chamber perfused with physiologic salt solution. Effluent from the chamber was collected for 8 hours, and 45Ca efflux curves were analyzed using compartmental analysis. When aortic segments were loaded and washed out in dinitrophenol, the slowest component of the efflux curve was less prominent; in high phosphate it was more prominent. The rate constant changes required to account for these data were primarily in the exchange between the cytosolic and slowest intracellular calcium compartment, suggesting that the slowest calcium compartment resolved during the 8-hour washout was mitochondrial. This compartment contained 5.4 +/- 3.2 nmol calcium/g wet wt. tissue. The calcium flux across its membranes was 0.32 +/- 0.04 nmol min-1g-1. Because this flux is much smaller than the plasma-membrane calcium flux, we suggest that, in normal physiological circumstances, plasma-membrane extrusion is more important for the removal of Ca from the smooth muscle cytosol than is uptake into this slow intracellular compartment.

Calcium exchange in the resting and electrically stimulated canine myocardium

45Ca2+ exchange was studied in small pieces of canine left ventricular free wall. The loss of 45Ca2+ from 45Ca2+ equilibrated tissue into ice-cold (4 degrees C) "wash" medium could be best fitted with a model containing a minimum of 3 compartments. 45Ca2+ uptake into and 45Ca2+ efflux from the most slowly exchanging compartment (compartment 3) at 37 degrees C allowed it to be subdivided into two fractions; a rapidly exchanging fraction (t1/2 approximately 1.25 min) and a slowly exchanging fraction (t1/2 approximately 50 min). The total Ca2+ content of compartment 3 was enhanced by isoproterenol but little affected by caffeine. The slow t1/2 for exchange of the Ca2+ in compartment 3 at 4 degrees C and its increased Ca2+ content following isoproterenol treatment suggest that this compartment contains some Ca2+ of intracellular origin. In addition, the finding that the rapidly exchanging part of compartment 3 could be preserved by cooling the tissue to 4 degrees C shows that rapidly exchanging Ca2+ compartments can be studied in superfused cardiac preparations using this technique. Action potentials, elicited by electrical stimulation of the tissue, caused large changes in the Ca2+ content of compartment 3 (up to 170 muM/kg) indicating that this postulated intracellular compartment may play a significant role in the normal contraction-relaxation cycle.

The effect of high K+ depolarization and verapamil on 45Ca2+ fluxes in the canine myocardium

The effects of high K+ depolarization and verapamil on Ca2+ uptake and the total intracellular Ca2+ content of canine ventricular muscle strips (approximately 0.5 mm thick) were investigated. High K+ (96 mM) increased Ca2+ uptake above control and maintained this enhanced uptake throughout a 90 second measuring period. Verapamil (5 X 10(-6) M) significantly (p less than 0.05) inhibited this high K+ stimulated uptake. However, verapamil (5 X 10(-6) M) also had a direct effect on Ca2+ fluxes, causing a significant increase in both Ca2+ uptake (p less than 0.001) and total intracellular Ca2+ content (p less than 0.001) in the resting tissue. Therefore, verapamil's apparent inhibition of high K+ stimulated Ca2+ uptake may have resulted from some mechanism other than Ca2+ channel blockade.

The role of calcium ion in luteal function in the rat

The regulatory role of calcium ion was investigated in isolated 10-day-old corpora lutea incubated in vitro. The corpora lutea were induced in immature rats by a single injection of PMSG (15 i.u.) on day 30. We examined the effect of various incubation conditions on the increase (about 7-fold) in cyclic AMP (cAMP) concentration by LH (5 micrograms/ml) and its reversal by PGF2 alpha (10 microM). In calcium-free medium (+0.5 mM EGTA) the stimulation by LH was only slightly impaired, and PGF2 alpha was fully effective in suppressing it. Similarly, both LH and PGF2 alpha acted normally in the presence of 100 microM verapamil, a blocker of calcium uptake. Trifluoperazine (TFP, 3-300 microM) a potent inactivator of calmodulin, did not interfere with the action of PGF2 alpha. The effect of LH was increased by TFP (30 and 300 microM); this was probably due to inhibition of calmodulin-dependent phosphodiesterase, since the increase of the response to LH by IBMX (0.5 mM) plus TFP (30 microM) was similar to that by IBMX alone. Finally, the uptake of radioactive calcium was not increased by PGF2 alpha in the absence or presence of LH. These results do not support the suggestion that calcium ion mediates the hormonal regulation of cAMP in the rat corpus luteum.

Effects of Na readmission on cellular 45Ca fluxes in Na-depleted guinea pig taenia coli

The removal of Na from the medium causes a cellular Ca uptake in the smooth muscle of the guinea pig taenia coli which is rapidly reversed if medium Na is readmitted. This net extrusion was characterized in tissues which were first Na-depleted in a zero-Na (sucrose) solution. Li was able to substitute for Na in mediating this effect. K was also able to mimic Na in this respect if the depolarization-mediated Ca influx caused by the isotonic K solution was blocked with 10(-5) M D -600. The net Ca extrusion upon Na readmission was due to a small decrease in Ca influx, as well as a marked increase in the transmembrane Ca efflux rate, as revealed by 45Ca washout experiments. The increased 45Ca efflux upon Na readmission could be mimicked by Li, K, choline and tris. We conclude that the Na/Ca-exchange hypothesis is insufficient to explain these data, in that both Ca extrusion and 45Ca efflux can be stimulated in the absence of a Na gradient, or in the absence of any monovalent cationic gradient. These observations are discussed in terms of a possible intracellular competition of Ca and monovalent cations for anionic binding sites, as well as with regard to a possible direct stimulation of a plasmalemmal CaATPase by monovalent cations.

Competitive membrane adsorption of Na+, K+, and Ca2+ in smooth muscle cells

A theory for Na+, K+ and Ca2+ competitive adsorption to a charged membrane is used to explain a number of experimental observations in smooth muscle. Adsorption is described by Langmuir isotherms for mono- and divalent cations which in turn are coupled in a self-consistent way to the bulk solution through the diffuse double layer theory and the Boltzman equations. We found that the dissociation constants for binding of Na+, K+ and Ca2+ in guinea pig taenia coli are ca. 0.009, 1.0, and 4 X 10(-8) M, respectively. Furthermore, the effect of a Ca2+ pump that maintains free surface Ca2+ concentration constant is investigated. A decrease in intracellular Na+ content results in an increased Ca2+ uptake; part of this uptake is due to an increase in surface-bound Ca2+ in an intracellular compartment which is in contact with the myofilaments. Variations in the amount of charge available to bind Ca2+ and the surface charge density are studied and their effect interpreted in terms of different pharmacological agents.

Effects of sodium gradient manipulation upon cellular calcium, 45Ca fluxes and cellular sodium in the guinea-pig taenia coli

1. Sucrose and choline were utilized as NaCl substitutes in order to investigate Na-Ca interactions in the smooth muscle of the guinea-pig taenia coli.2. Progressive substitution of NaCl by sucrose caused a progressive increase in cellular exchangeable Ca. This uptake, which amounted to about 300 mumole Ca/kg tissue upon total Na replacement, reached a plateau within 20 min. Complete substitution of NaCl by choline chloride caused cellular Ca to increase rapidly to an initial peak, and then decrease to a stable plateau which was also about 300 mumole/kg above control.3. Replacement of NaCl by either sucrose or choline chloride caused a transient increase in the Ca influx rate, which was measured using a 3 min pulse labelling with (45)Ca. This increase was more pronounced in choline chloride.4. NaCl substitution by either sucrose or choline chloride caused a decrease in the (45)Ca efflux rate. Two exponential components of transmembrane (45)Ca efflux were found in control and Na-free media.5. Treatment of tissues with 3 x 10(-5)m-ouabain did not significantly affect the cellular Ca content after 80 min, at which time the Na and K gradients were largely dissipated.6. Removal of medium K caused a slower dissipation of the Na and K gradients. This treatment decreased cellular Ca, did not affect the Ca influx rate, and increased the (45)Ca efflux rate.7. Tissues were incubated in depolarizing media containing 10(-4)m-ouabain in order to remove the Na gradient. Subsequent measurement of cellular Na indicated the absence of a significant fraction of bound Na.8. The ratio [Na](o)/[Na](i) had a value of 6.3 in control medium, and decreased as [Na](o) was progressively lowered by sucrose substitution, reaching a value of < 1 in a medium containing 5 mm-Na.9. These experiments provide evidence that a Na-Ca exchange carrier does not play an important role in regulation of tension in this muscle, and also indicate that the Ca gradient is not solely dependent on the Na gradient in guinea-pig taenia coli.

Ca2+ movements in smooth muscle

We describe the Ca2+ movements in smooth muscle cells at rest and during activation and relaxation as deduced from transplasmalemmal Ca2+ fluxes and contractile respnses. The general picture which emerges is: the resting cell has a [Ca2+]cyt below 10(-7) M and large gradients are poised across both the cell membrane and intracellular membranes. Excitation opens up Ca2+ channels which are linked to receptors and, if depolarization occurs, to other channels not linked to specific receptors but capable of sensing the membrane potential. Receptor activation also leads to release of Ca2+ from a limited intracellular Ca2+ pool which is superficially located because it has to be refilled from the outside. Relaxation is effected by Ca2+ accumulation by another intracellular Ca2+ pool, very likely sarcoplasmic reticulum, which does not release Ca2+ during activation. The sarcoplasmic reticulum Ca2+ pump can also decrease initial activation of the myofilaments. Elevation of cAMP levels may inhibit contraction by stimulating the sarcoplasmic reticulum Ca-ATPase. An enormous amount of research is still required to prove the above scheme and to localize and quantitate the various intracellular Ca2+ pools.