The inhibitory action of caffeine on calcium currents in isolated intestinal smooth muscle cells

Pharmacological evidence that GABA-induced relaxation of rat proximal duodenum longitudinal muscle depends on NKCC cotransporter activity and Ca2+ influx

γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in adult central nervous system (CNS) synapses, but it excites immature CNS neurons as well as neurons in the myenteric plexus. The present work aimed to determine whether GABA-induced nonadrenergic, noncholinergic (NANC) neuronal-mediated relaxation of the rat duodenum is dependent on the activity of Na⁺ K⁺ Cl^- cotransporters (NKCC) and requires calcium influx. In the presence of guanethidine (3 µmol/L), atropine (3 µmol/L), and indomethacin (1 µmol/L), relaxations induced by GABA (100 µmol/L), KCl (5-10 mmol/L) and electrical field stimulation (1-8 Hz, 2 ms, 60 V), but not those induced by bradykinin (10-100 nmol/L) were abolished by lidocaine (300 µmol/L). However, only GABA-induced relaxations were reduced in a concentration-dependent manner by the NKCC1/2 inhibitors bumetanide (0.1-1 µmol/L) and furosemide (1-10 µmol/L). GABA-induced NANC neuronal relaxation was abolished by bicuculline (30 µmol/L) and inhibited by N-nitroarginine methyl ester (l-NAME, 300 µmol/L). The ω-conotoxin GVIA (1 µmol/L), which acts exclusively on neuronal Ca_V2 channels, but not on smooth muscle voltage-gated Ca²⁺ Ca_V1 channels, and nonselective blockers of these channels (verapamil 100 nmol/L and ruthenium red 10 µmol/L), reduced GABA-induced relaxations. These results showed that the activation of GABA_A receptors induces NANC nitrergic neuronal relaxations in the rat duodenum, which depend on NKCC activity and Ca_V2 channel activation, suggesting that this phenomenon results from neuronal depolarization promoted by Cl^- efflux through GABA_A receptors, with subsequent Ca²⁺ influx and nitric oxide release.

Coffee and Endothelial Function: A Coffee Paradox?

Coffee is a popular beverage throughout the world. Coffee contains various chemical compounds (e.g., caffeine, chlorogenic acids, hydroxyhydroquinone, kahweol, cafestol, and complex chemical mixtures). Caffeine is also the most widely consumed pharmacological substance in the world and is included in various beverages (e.g., coffee, tea, soft drinks, and energy drinks), products containing chocolate, and drugs. The effects of coffee and caffeine on cardiovascular diseases remain controversial. It is well known that there are J-curve-type or U-curve-type associations of coffee consumption with cardiovascular events including myocardial infarction and stroke. However, there is little information on the direct and indirect effects of coffee consumption on endothelial function in humans. It is likely that the coffee paradox or caffeine paradox exists the association of coffee intake with cardiovascular diseases, cardiovascular outcomes, and endothelial function. This review focusses on the effects of coffee and caffeine on endothelial function from molecular mechanisms to clinical perspectives.

Role of mitochondria in contraction and pacemaking in the mouse uterus

BACKGROUND AND PURPOSE

Uterine spontaneous contraction and pacemaking are poorly understood. This study investigates the role of the mitochondrial Ca(2+) store in uterine activity.

EXPERIMENTAL APPROACH

We investigated the effects of mitochondrial and sarco-endoplasmic reticulum (SER) inhibitors on contraction, membrane potential (Vm) and cytosolic Ca(2+) concentration ([Ca(2+) ](c) ) in longitudinal smooth muscle of the mouse uterus.

KEY RESULTS

The mitochondrial agents rotenone, carbonylcyanide-3-chlorophenylhydrazone (CCCP), 7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one (CGP37157) and kaempferol decreased the force of contractions. The ATP synthase inhibitor oligomycin had no significant effect. The effects of these agents were compared with those of SER inhibitors cyclopiazonic acid (CPA), 2-amino ethoxyphenylborate (2-APB) and caffeine. All agents, except CPA and oligomycin, decreased contractile force. CPA and CCCP transiently increased contraction frequency, which returned to control levels, whereas rotenone, CGP37157, kaempferol and 2-APB decreased frequency and caffeine had no significant effect. Application of the mitochondrial agents when CPA functionally inhibited stores did not change contraction frequency but, with the exception of kaempferol, decreased force. CCCP caused depolarization and maintained increase in [Ca(2+) ](c) or depolarization/transient hyperpolarization and transient increase in [Ca(2+) ](c) for oestrus and di-oestrus tissues respectively. Rotenone caused hyperpolarization and maintained increase in [Ca(2+) ](c) . CGP37157 and kaempferol caused hyperpolarization but no measurable change in [Ca(2+) ](c) . Application of a range of K(+) channel blockers indicated a role of Ca(2+) -activated K(+) (K(Ca) ) channels in the CCCP- and CGP37157-induced actions.

CONCLUSIONS AND IMPLICATIONS

Mitochondria have a modulatory role on uterine contractions, with mitochondrial inhibition reducing contraction amplitude and pacemaker frequency by changes in Vm, [Ca(2+) ](c) and/or Ca(2+) influx.

Caffeine inhibits InsP3 responses and capacitative calcium entry in canine pulmonary arterial smooth muscle cells

Caffeine is a well described and characterized ryanodine receptor (RyR) activator. Previous evidence from independent research studies also indicate caffeine inhibits InsP3 receptor functionality, which is important to activation of capacitative Ca2+ entry (CCE) in some cell types. In addition, RyR activation elicits excitatory-coupled Ca2+ entry (ECCE) in skeletal muscle myotubes. Recent studies by our group show that canine pulmonary arterial smooth muscle cells (PASMCs) have functional InsP3 receptors as well as RyRs, and that CCE is dependent on InsP3 receptor activity. The potential for caffeine to activate ECCE as well as inhibit InsP3 receptor function and CCE was examined using fura-2 fluorescent imaging in canine PASMCs. The data show caffeine causes transient as well as sustained cytosolic Ca2+ increases, though this is not due to CCE or ECCE activity as evidenced by a lack of an increase in Mn2+ quench of fura-2. The experiments also show caffeine reversibly inhibits 5-HT elicited-InsP3 mediated Ca2+ responses with an IC50 of 6.87x10(-4) M and 10 mM caffeine fully inhibits CCE. These studies provide the first evidence that caffeine is an inhibitor of InsP3 generated Ca2+ signals and CCE in PASMCs.

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.

Regulation of muscarinic cationic current in myocytes from guinea-pig ileum by intracellular Ca2+ release: a central role of inositol 1,4,5-trisphosphate receptors

The dynamics of carbachol (CCh)-induced [Ca(2+)](i) changes was related to the kinetics of muscarinic cationic current (mI(cat)) and the effect of Ca(2+) release through ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (IP(3)Rs) on mI(cat) was evaluated by fast x-y or line-scan confocal imaging of [Ca(2+)](i) combined with simultaneous recording of mI(cat) under whole-cell voltage clamp. When myocytes freshly isolated from the longitudinal layer of the guinea-pig ileum were loaded with the Ca(2+)-sensitive indicator fluo-3, x-y confocal imaging revealed CCh (10 microM)-induced Ca(2+) waves, which propagated from the cell ends towards the myocyte centre at 45.9 +/- 8.8 microms(-1) (n = 13). Initiation of the Ca(2+) wave preceded the appearance of any measurable mI(cat) by 229 +/- 55 ms (n = 7). Furthermore, CCh-induced [Ca(2+)](i) transients peaked 1.22 +/- 0.11s (n = 17) before mI(cat) reached peak amplitude. At -50 mV, spontaneous release of Ca(2+) through RyRs, resulting in Ca(2+) sparks, had no effect on CCh-induced mI(cat) but activated BK channels leading to spontaneous transient outward currents (STOCs). In addition, Ca(2+) release through RyRs induced by brief application of 5 mM caffeine was initiated at the cell centre but did not augment mI(cat) (n = 14). This was not due to an inhibitory effect of caffeine on muscarinic cationic channels (since application of 5 mM caffeine did not inhibit mI(cat) when [Ca(2+)](i) was strongly buffered with Ca(2+)/BAPTA buffer) nor was it due to an effect of caffeine on other mechanisms possibly involved in the regulation of Ca(2+) sensitivity of muscarinic cationic channels (since in the presence of 5 mM caffeine, photorelease of Ca(2+) upon cell dialysis with 5 mM NP-EGTA/3.8 mM Ca(2+) potentiated mI(cat) in the same way as in control). In contrast, IP(3)R-mediated Ca(2+) release upon flash photolysis of "caged" IP(3) (30 microM in the pipette solution) augmented mI(cat) (n = 15), even though [Ca(2+)](i) did not reach the level required for potentiation of mI(cat) during photorelease of Ca(2+) (n = 10). Intracellular calcium stores were visualised by loading of the myocytes with the low-affinity Ca(2+) indicator fluo-3FF AM and consisted of a superficial sarcoplasmic reticulum (SR) network and some perinuclear formation, which appeared to be continuous with the superficial SR. Immunostaining of the myocytes with antibodies to IP(3)R type 1 and to RyRs revealed that IP(3)Rs are predominant in the superficial SR while RyRs are confined to the central region of the cell. These results suggest that IP(3)R-mediated Ca(2+) release plays a central role in the modulation of mI(cat) in the guinea-pig ileum and that IP(3) may sensitise the regulatory mechanisms of the muscarinic cationic channels gating to Ca(2+).

Depletion of Ca2+ from intracellular stores potentiates spontaneous contractions of the rat portal vein

Spontaneous contractions of the rat portal vein were potentiated in magnitude by phenylephrine, cyclopiazonic acid, ryanodine or caffeine. All these drugs can deplete Ca2+ from intracellular stores, which stimulates store-operated cation entry in some tissues. The possibility that depletion of Ca2+ from intracellular stores potentiates the spontaneous contractions was therefore investigated using functional experiments. Phenylephrine or cyclopiazonic acid was added to tissues in Ca2+-free Krebs solution, followed by a 30-min washout. After addition of extracellular Ca2+, the spontaneous contractions were potentiated. This showed the stimulus for potentiating the contractions remained so long as intracellular Ca2+ stores were depleted. Following phenylephrine washout in normal Krebs solution, potentiation of the spontaneous contractions was attenuated with time. This attenuation was abolished by the protein kinase C inhibitor calphostin C. These results show depletion of Ca2+ from intracellular stores potentiates spontaneous contractions of the portal vein. Protein kinase C may inhibit this mechanism.

The effects of caffeine on ATP-sensitive K(+) channels in smooth muscle cells from pig urethra

The effects of caffeine on both levcromakalim-induced macroscopic and unitary currents in pig proximal urethra were investigated by the use of patch-clamp techniques (conventional whole-cell configuration and cell-attached configuration). The effects of caffeine were also examined on currents in inside-out patches of COS7 cells expressing carboxy terminus truncated inwardly rectifying K(+) channel (Kir6.2) subunits (i.e. Kir6.2DeltaC36) which form ATP-sensitive K(+) channels (K(ATP) channels). In conventional whole-cell configuration, the levcromakalim (100 microM)-induced inward current (symmetrical 140 mM K(+) conditions) was inhibited by caffeine (> or =1 mM) at a holding potential of -50 mV. In contrast, ryanodine (10 microM) caused no significant inhibitory effect on the gradual decay of the levcromakalim-induced current at -50 mV. The amplitude of the 30 microM levcromakalim-induced current was enhanced by 3-isobutyl-1-methylxanthine (IBMX, 100 microM). In cell-attached configuration, the levcromakalim-induced K(+) channel openings were inhibited by subsequent application of 10 mM caffeine, decreasing the channel open probability at -50 mV. Reverse transcriptase-polymerase chain reaction (RT - PCR) analysis revealed the presence of Kir6.2 transcript in pig urethra. Caffeine (> or =3 mM) inhibited the channel activity of Kir6.2DeltaC36 expressed in COS7 cells (3 mM caffeine, 65+/-6%, n=4; 10 mM caffeine, 29+/-2%, n=4). These results suggest that caffeine can inhibit the activity of K(ATP) channels through a direct blocking effect on the pore-forming Kir subunit.

Investigation of the role of intracellular Ca(2+) stores in generation of the muscarinic agonist-induced slow afterdepolarization (sADP) in guinea-pig olfactory cortical neurones in vitro

1. Intracellular recordings were made from guinea-pig olfactory cortical brain slice neurones to assess the possible role of intracellular Ca(2+) stores in the generation of the slow post-stimulus afterdepolarization (sADP) and its underlying tail current (I(ADP)), induced by muscarinic receptor activation. 2. Caffeine or theophylline (0.5 - 3 mM) reduced the amplitude of the I(ADP) (measured under 'hybrid' voltage clamp) induced in the presence of the muscarinic agonist oxotremorine-M (OXO-M, 10 microM) by up to 96%, without affecting membrane properties or muscarinic depolarization of these neurones. 3. The L-type Ca(2+) channel blocker nifedipine (1, 10 microM) also inhibited I(ADP) (by up to 46%), while ryanodine (10 microM) (a blocker of Ca(2+) release from internal stores) produced a small ( approximately 10%) reduction in I(ADP) amplitude; however, neither 10 microM dantrolene (another internal Ca(2+) release blocker) nor the intracellular Ca(2+) store re-uptake inhibitors thapsigargin (3 microM) or cyclopiazonic acid (CPA, 15 microM) affected I(ADP) amplitude. 4. IBMX (100 microM), a phosphodiesterase inhibitor, also had no effect on I(ADP). Furthermore, inhibition of I(ADP) by caffeine was not reversed by co-application of 100 microM adenosine. 5. Caffeine (3 mM) or nifedipine (10 microM) reduced the duration of presumed Ca(2+) spikes revealed by intracellular Cs(+) loading. When applied in combination, nifedipine and caffeine effects were occlusive, rather than additive, suggesting a common site of action on L-type calcium channels. 6. We conclude that Ca(2+)-induced Ca(2+) release (CICR) from internal stores does not contribute significantly to muscarinic I(ADP) generation in olfactory cortical neurones. However caffeine and theophylline, which enhance CICR in other systems, blocked I(ADP) induction. We suggest that this action might involve a combination of L-type voltage-gated Ca(2+) channel blockade, and a direct inhibitory action on the putative I(ADP) K(+) conductance.

The influence of phenylephrine outward potassium currents in single smooth muscle cells from the rabbit mesenteric artery

In mesenteric artery smooth muscle cells, depolarizing voltage steps activated outward K+ currents whose amplitude was decreased by about 20% with phenylephrine (1-10 microM: n = 14 cells). Attenuation of outward current was only partly dependent on [Ca2+]i, because it persisted, although reduced, with 10 mM BAPTA in the patch pipette and was abolished in the presence of 3 mM 3,4-diaminopyridine (n = 13). In outside-out patches, phenylephrine did not exert any direct effect on the unitary current amplitude or open probability of large conductance K+ channels. Outward current was significantly increased (>100% in both cases) by 10 mM caffeine, presumably owing to the release of internal Ca2+ stores. With 10 mM BAPTA in the pipette, the only response to caffeine was a small decrease (9 +/- 3.7%, n = 10) in the K+ current. These observations show that a minor effect of phenylephrine is to reduce outward K+ current (probably Kv) in mesenteric cells.

Regulation of intracellular ATP concentration under conditions of reduced ATP consumption in pancreatic islets

ATP is the most important factor in glucose-induced insulin secretion in pancreatic beta-cells, but examination of intracellular differences in ATP concentration is difficult because ATP production and consumption occur simultaneously. In the present study, we measured the ATP concentration under the condition of a reduced ATP requirement by omitting extracellular Ca(2+) and inhibiting Na-K ATPase. The ATP concentration in islets incubated with 16.7 mM glucose in the absence of Ca(2+) for 30 min was increased by about 1. 9-fold more than in the presence of Ca(2+). The increment was extracellular Ca(2+)-dependent, and was completely abolished by the metabolic inhibitors dinitrophenol and iodoacetic acid. The Ca channel blockers including nitrendipine and Ni(2+) did not affect the ATP concentration in islets incubated with 16.7 mM glucose in the presence of Ca(2+). However, when thapsigargin and suramin, inhibitors of Ca-ATPase at the endoplasmic reticulum, were added to Ca channel blockers in the presence of ambient Ca(2+), the intraislet ATP content was increased, similarly to that under Ca-free conditions. But thapsigargin did not further augment the ATP concentration in the islet with 16.7 mM glucose in the absence of Ca(2+). On the other hand, the suppression of Na-K ATPase by ouabain rather reduced the ATP concentration augmented by omission of extracellular Ca(2+). In addition, vanadate, a blocker of Ca-ATPase at the plasma membrane, failed to increase the ATP concentration in the islets. These data suggest that the increment of ATP concentration in the absence of Ca(2+) is attributable to the reduced ATP requirement due to stopping of the Ca-ATPase activity at the endoplasmic reticulum, and that the intracellular ATP concentration is differently regulated by Na-K ATPase at plasma membrane and by Ca-ATPase at endoplasmic reticulum.

Contribution of nitric oxide and K+ channel activation to vasorelaxation of isolated rat aorta induced by procaine

The endothelium-dependent and -independent relaxant effect of procaine was examined in isolated rat aortic rings. Procaine induced relaxation of arteries precontracted with phenylephrine or with 60 mM K+ in a concentration-dependent manner (0.01-3 mM). Procaine (1 mM) inhibited the transient contraction induced by caffeine (10 mM) in Ca2+-free Krebs solution. Removal of the endothelium caused a rightward shift of the concentration-response curve for procaine. N(G)-Nitro-L-arginine (L-NNA, 10-100 microM), N(G)-nitro-L-arginine methyl ester (L-NAME, 100 microM) and methylene blue (1-10 microM) significantly attenuated the procaine-induced relaxation without affecting the maximal response. L-Arginine (1 mM) partially but significantly antagonized the effect of L-NAME (100 microM). Pretreatment of endothelium-intact aortic rings with procaine (1 mM) or with acetylcholine (10 microM) significantly elevated the tissue contents of cyclic GMP and this increase was inhibited in the presence of 100 microM L-NNA. Tetrapentylammonium ions (1-3 microM) reduced the procaine-induced relaxation in both endothelium-intact and -denuded arteries. Tetrapentylammonium ions (3 microM) did not affect the procaine-induced relaxation of 60 mM K+-contracted arteries. Tetraethylammonium ions (3 mM) inhibited the procaine-induced relaxation. In contrast, iberiotoxin (100 nM), glibenclamide (3 microM), 4-aminopyridine (3 mM) and indomethacin (10 microM) had no effect. These results indicate that the procaine-induced relaxation may be mediated through multiple mechanisms. A substantial portion of the procaine-induced relaxation in rat aorta was caused by nitric oxide but not by other endothelium-derived factors. The activation of tetrapentylammonium- and tetraethylammonium-sensitive K+ channels contributes in part to the procaine-induced vasorelaxation. Besides, procaine may directly inhibit both external Ca2+ entry and internal Ca2+ release in aortic smooth muscle cells.

Ruthenium red as an effective blocker of calcium and sodium currents in guinea-pig isolated ventricular heart cells

1. The effect of ruthenium red on calcium and sodium currents was studied in guinea-pig isolated ventricular heart cells with the whole cell patch-clamp technique. 2. Ruthenium red very efficiently blocked the L-type calcium current in a dose-dependent manner. A significant block was observed for concentrations as low as 0.3 microM. Analysis of the dose-response curve with the logistic equation indicated an EC50 of 0.8 microM, a maximum inhibition of 85% reached at 5 microM, and a coefficient of 2.37. 3. There was no shift in the voltage-dependence of the Ca current activation, nor in that of its steady-state inactivation determined with a 1 s prepulse. However, removal of Ca current inactivation at positive voltage was considerably reduced in the presence of concentrations of ruthenium red above 1 microM. A slowing of the time-course of inactivation of the Ca current was also observed. 4. At 10 microM, a concentration generally used to block the sarcoplasmic Ca release channels or the mitochondrial Ca uptake, ruthenium red blocked 26.7+/-4.3% (n=8) of the sodium current, and slowed its inactivation time-course. No effect was observed on the voltage-dependence of the current activation or inactivation. The peak sodium current was also decreased at a 10 times lower concentration by 7.6+/-2.7% (n=3). 5. Thus, at concentrations used to assess intracellular Ca movements, ruthenium red induced in heart cells a significant block of both Ca and Na channels.

Effects of caffeine on the influx of extracellular calcium in GH4C1 pituitary cells

Caffeine increases intracellular Ca2+ concentrations ([Ca2+]i) in a variety of cell types by triggering the mobilization of Ca2+ from intracellular Ca2+ stores. Caffeine also can change [Ca2+]i by affecting Ca2+ influx through voltage-operated Ca2+ channels (VOCCs). In the present study, we investigated the effects of caffeine on Ca2+ entry in GH4C1 pituitary cells. Pretreatment of the cells with caffeine attenuated the high K+-evoked influx of 45Ca2+ in a dose-dependent manner. This inhibition was not secondary to the caffeine-evoked elevation of [Ca2+]i because caffeine was able to inhibit VOCCs also in the presence of the intracellular Ca2+ chelator BAPTA. However, the inhibitory effect of caffeine on 45Ca2+ entry appeared to be dependent on the degree of depolarization of the plasma membrane. Only in cells depolarized with relatively high concentrations of K+ (20, 35, and 50 mM) was the caffeine-induced inhibition observed. A similar inhibitory effect of caffeine on the high K+-evoked calcium and barium entry was observed in experiments using Fura 2. Neither IBMX, forskolin nor dibutyryl cAMP reduced the enhanced [Ca2+]i induced by 50 mM K+, suggesting that the effect of caffeine was not due to increased intracellular cAMP. Furthermore, high doses of caffeine inhibited the plateau level of the TRH-induced increase in [Ca2+]i, which is caused partly by influx of Ca2+ through VOCCs. The inhibitory effect of caffeine was, in part, due to an hyperpolarization of the plasma membrane observed at high doses of caffeine. On the other hand, low doses of caffeine enhanced depolarization-evoked Ba2+ entry as well as the TRH-evoked plateau level of [Ca2+]i. We conclude that caffeine has a dual effect on Ca2+ entry through activated VOCCs in GH4C1 cells: at low concentrations caffeine enhances Ca2+ entry, whereas high concentrations of caffeine block Ca2+ entry.

Voltage-dependent calcium currents and cytosolic calcium in equine airway myocytes

1. The relationship between voltage-dependent calcium channel current (I(Ca)) and cytosolic free calcium concentration ([Ca2+]i) was studied in fura-2 AM-loaded equine tracheal myocytes at 35 degrees C and 1.8 mM Ca2+ using the nystatin patch clamp method. The average cytosolic calcium buffering constant was 77 +/- 3 (n = 14), and the endogenous calcium buffering constant component is likely to be between 15 and 50. 2. I(Ca) did not evoke significant calcium-induced calcium release (CICR) since (i)[Ca2+]i scaled with the integrated I(Ca) over the full voltage range of evoked calcium currents, (ii) increases in [Ca2+]i associated with I(Ca) were consistent with cytoplasmic buffering of calcium ions entering through voltage-dependent calcium channels (VDCCs) only, (iii) there was a fixed instantaneous relationship between transmembrane calcium flux (J(Ca)) and the change in cytosolic free calcium concentration (delta [Ca2+]i) during I(Ca), (iv) caffeine (8 mM) triggered 8-fold higher calcium transients than I(Ca), and (v) I(Ca) evoked following release of intracellular calcium by caffeine resulted in an equivalent delta[Ca2+]i-J(Ca) relationship. 3. The time constant (T) for the decay in [Ca2+]i was 8.6 +/- 1.5 s (n = 8) for single steps and 8.6 +/- 1.1 s (n = 13) following multiple steps that increased [Ca2+]i to much higher levels. Following application of caffeine (8 mM), however, [Ca2+]i decay was enhanced (T = 2.0 +/- 0.2 s, n = 3). The rate of [Ca2+]i decay was not voltage dependent, was not decreased in the absence of extracellular Na+ ions, and no pump current was detected. 4. We conclude that under near physiological conditions, neither CICR nor Na(+)-Ca2+ exchange play a substantial role in the regulation of I(Ca)-induced increases in [Ca2+]i, and that, even following release of intracellular calcium by caffeine, Na(+)-Ca2+ exchange does not play an appreciable role in the removal of calcium ions from the cytosol.

Voltage-dependent suppression of calcium current by caffeine in single smooth muscle cells of the guinea-pig urinary bladder

The suppressive action of caffeine on L-type Ca current (Ica) in smooth muscle cells of the guinea-pig urinary bladder was investigated using the whole-cell patch clamp technique. Caffeine (5-30 mM) suppressed Ica, the effect having two phases: a rapid and transient suppression of Ica, which was followed by a sustained suppression. When intracellular Ca2+ was strongly buffered by the Ca2+ chelator EGTA (20 mM) or BAPTA (5 mM) in the patch pipette, the transient suppression of Ica was abolished, whereas the sustained effect remained. Similarly, inclusion of both 10 mM procaine and 1 mg/ml heparin in the patch pipette blocked the transient suppression of Ica, but did not block the sustained effect. The degree of the sustained effect of caffeine on Ica was dose-dependent with a kd of 20 mM. Application of the cyclic AMP analogue, 8-bromo-cyclic AMP (100 microM) or forskolin (10 microM) to the bath failed to mimick the sustained suppression of Ica, suggesting that inhibition of phosphodiesterase activity was not involved in the caffeine action. The steady-state activation curve remained unchanged by 10 mM caffeine but the steady-state inactivation curve was significantly shifted in the negative direction by 15.6 mV in 1.8 mM Ca2+ solution or by 10 mV in 1.8 mM Ba2+ solution. From these results it appears that caffeine inhibits L-type Ica via two mechanisms: (1) it releases Ca2+ from an internal store causing a transient Ca2+ -mediated inactivation of the Ca channel; (2) it inhibits Ca channel via a mechanism that does not require such a Ca2+ release. It is possible that caffeine suppresses Ica through a preferential binding to the inactivated state of L-type Ca channel.

Caffeine-evoked, calcium-sensitive membrane currents in rabbit aortic endothelial cells

1. Single cell photometry and whole-cell patch clamp recording were used to study caffeine-induced intracellular Ca2+ signals and membrane currents, respectively, in endothelial cells freshly dissociated from rabbit aorta. 2. Caffeine (5 mM) evoked a transient increase in [Ca2+]i in fura-2-loaded endothelial cells. Pretreatment of cells with 10 microM ryanodine did not alter resting [Ca2+]i but irreversibly inhibited the caffeine-induced rise in [Ca2+]i. The caffeine-induced increase in [Ca2+]i was not attenuated by the removal of extracellular Ca2+ and did not stimulate the rate of Mn2+ quench of fura-2 fluorescence. 3. Bath application of caffeine evoked a dose- and voltage-dependent outward current. The rate of onset and amplitude of the caffeine-evoked outward current increased with higher caffeine concentrations and membrane depolarization. The relationship between caffeine-evoked current amplitude and membrane potential was non linear, suggesting that the channels underlying the current are voltage-sensitive. 4. In the absence of extracellular Ca2+, the amplitude of the caffeine-evoked outward current was reduced by approximately 50% but the duration of the current was prolonged compared to that observed in the presence of external Ca2+. Ca(2+)-free external solutions produced an unexpected increase in both the frequency and amplitude of spontaneous transient outward currents (STOCs). 5. Inclusion of heparin (10 micrograms ml-1) in the patch pipette abolished the acetylcholine (ACh)-induced outward current but failed to inhibit either STOCs or the caffeine-evoked outward current in native endothelial cells. In the absence of extracellular Ca2+, heparin did not affect either STOCs or the caffeine-induced outward current. 6. Externally applied tetraethylammonium ions (TEA, 3-10mM) reversibly inhibited unitary Ca2+-activated K+ currents and STOCs in endothelial cells but failed to inhibit completely the outward current evoked by 20 mM caffeine.7. Bath application of 0.1 mM zinc ion (Zn2+), a chloride channel blocker, did not affect unitary currents or STOCs but reduced the amplitude of the caffeine-evoked current by >75% compared to control. Replacement of extracellular NaCl with Na gluconate also reduced the amplitude of the caffeine-induced outward current. Bath application of 0.1 mM Zn2+ and 10 mM TEA completely blocked the caffeine-evoked outward current in endothelial cells.8. Caffeine-induced Ca2+ release from intracellular stores evokes a transient rise in [Ca2+1, which is correlated with a large, transient outward current. The ionic dependence and inhibition of the caffeine sensitive current by TEA and Zn2+ suggests that Ca2+-activated K+ and Cl- conductances contribute to the caffeine response in rabbit aortic endothelial cells.

Efficacy of peak Ca2+ currents (ICa) as trigger of sarcoplasmic reticulum Ca2+ release in myocytes from the guinea-pig coronary artery

1. Increments in cytosolic Ca2+ concentration (delta[Ca2+]c) were measured in single smooth muscle cells from guinea-pig coronary artery together with the density of peak Ca2+ currents (ICa) in response to clamp steps from -50 to 0 mV. The comparison of depolarization- with caffeine-induced delta[Ca2+]c was used to define the efficacy by which ICa can trigger Ca2+ release from the sarcoplasmic reticulum (SR). 2. At 2.5 mM extracellular calcium concentration ([Ca2+]o), depolarization induced a rapid rise of delta[Ca2+]c followed by a slow creep. Peak [Ca2+]c occurred within ca 30 s and could be followed by an undershoot and a second rise in [Ca2+]c. The creep was blocked by ryanodine but was insensitive to block of InsP3 receptors with heparin. The creep was not observed in Cs(+)-filled cells. After disappearance of the creep, a tonic delta[Ca2+]c became unmasked. 3. At 2.5 mM [Ca2+]o, peak ICa was -0.80 +/- 0.17 microA cm-2. delta[Ca2+] peaked at the end of the 6 s pulse at 202 +/- 98 nM while caffeine-induced delta[Ca2+]c peaked at 1330 +/- 410 nM. The ratio of depolarization- to caffeine-induced delta[Ca2+]c was 10 +/- 6%. 4. In media containing 10 mM [Ca2+]o plus 1 microM Bay K 8644, peak ICa was -2.6 +/- 1.1 microA cm-2 and delta[Ca2+]c peaked within 2.5 s at 451 +/- 194 nM. Paired measurements yielded the ratio of depolarization- to caffeine induced delta[Ca2+]c as 30 +/- 10%. Depolarization-induced delta[Ca2+]c was nearly blocked by caffeine and reduced by ryanodine to 30%, suggesting the contribution of Ca2+ release from caffeine- and ryanodine-sensitive Ca2+ stores. 5. Trypsin (1 mg ml-1) in the electrode solution (10 mM [Ca2+]o plus 1 microM Bay K 8644) increased peak ICa up to 12.5 microA cm-2. ICa induced a delta[Ca2+]c of 990 +/- 210 nM and was accompanied by a 'hump' of IK,Ca. When applied briefly after peak delta[Ca2+]c, caffeine increased [Ca2+]c only moderately. The results suggest that a peak ICa can trigger a synchronized whole-cell Ca2+ release only if ICa is strongly augmented. 6. Amplitude and rate of rise of delta[Ca2+]c were graded by test step potentials along a bell-shaped voltage-dependent curve, similar to that of L-type ICa. Steps to +80 mV induced no delta[Ca2+]c when the electrode solution contained 10 mM Na+. However, with 150 mM intrapipette Na+, pulses to +80 mV induced delta[Ca2+]c.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of caffeine on cytoplasmic free Ca2+ concentration in pancreatic beta-cells are mediated by interaction with ATP-sensitive K+ channels and L-type voltage-gated Ca2+ channels but not the ryanodine receptor

In the pancreatic beta-cell, an increase in the cytoplasmic free Ca2+ concentration ([Ca2+]i) by caffeine is believed to indicate mobilization of Ca2+ from intracellular stores, through activation of a ryanodine receptor-like channel. It is not known whether other mechanisms, as well, underlie caffeine-induced changes in [Ca2+]i. We studied the effects of caffeine on [Ca2+]i by using dual-wavelength excitation microfluorimetry in fura-2-loaded beta-cells. In the presence of a non-stimulatory concentration of glucose, caffeine (10-50 mM) consistently increased [Ca2+]i. The effect was completely blocked by omission of extracellular Ca2+ and by blockers of the L-type voltage-gated Ca2+ channel, such as D-600 or nifedipine. Depletion of agonist-sensitive intracellular Ca2+ pools by thapsigargin did not inhibit the stimulatory effect of caffeine on [Ca2+]i. Moreover, this effect of caffeine was not due to an increase in cyclic AMP, since forskolin and 3-isobutyl-1-methylxanthine (IBMX) failed to raise [Ca2+]i in unstimulated beta-cells. In beta-cells, glucose and sulphonylureas increase [Ca2+]i by causing closure of ATP-sensitive K+ channels (KATP channels). Caffeine also caused inhibition of KATP channel activity, as measured in excised inside-out patches. Accordingly, caffeine (> 10 mM) induced insulin release from beta-cells in the presence of a non-stimulatory concentration of glucose (3 mM). Hence, membrane depolarization and opening of voltage-gated L-type Ca2+ channels were the underlying mechanisms whereby the xanthine drug increased [Ca2+]i and induced insulin release. Paradoxically, in glucose-stimulated beta-cells, caffeine (> 10 mM) lowered [Ca2+]i. This effect was due to the fact that caffeine reduced depolarization-induced whole-cell Ca2+ current through the L-type voltage-gated Ca2+ channel in a dose-dependent manner. Lower concentrations of caffeine (2.5-5.0 mM), when added after glucose-stimulated increase in [Ca2+]i, induced fast oscillations in [Ca2+]i. The latter effect was likely to be attributable to the cyclic AMP-elevating action of caffeine, leading to phosphorylation of voltage-gated Ca2+ channels. Hence, in beta-cells, caffeine-induced changes in [Ca2+]i are not due to any interaction with intracellular Ca2+ pools. In these cells, a direct interference with KATP channel- and L-type voltage-gated Ca(2+)-channel activity is the underlying mechanism by which caffeine increases or decreases [Ca2+]i.

Effects of caffeine on intracellular calcium, calcium current and calcium-dependent potassium current in anterior pituitary GH3 cells

Caffeine elicits physiological responses in a variety of cell types by triggering the mobilization of Ca2+ from intracellular organelles. Here we investigate the effects of caffeine on intracellular Ca2+ concentration ([Ca2+]i) and ionic currents in anterior pituitary cells (GH3) cells. Caffeine has a biphasic effect on Ca(2+)-activated K+ current [IK(Ca)]: it induces a transient increase superimposed upon a sustained inhibition. While the transient increase coincides with a rise in [Ca2+]i, the sustained inhibition of IK(Ca) is correlated with a sustained inhibition of the L-type Ca2+ current. The L-type Ca2+ current is also inhibited by other agents that mobilize intracellular Ca2+, including thyrotropin releasing hormone (TRH) and ryanodine, but in a matter distinct from caffeine. Unlike the caffeine effect, the TRH-induced inhibition "washes-out" under whole-cell patch-clamp conditions and is eliminated by intracellular Ca2+ chelators. Likewise, the ryanodine-induced inhibition desensitizes while the caffeine-induced inhibition does not. Simultaneous [Ca2+]i and Ca2+ current measurements show that caffeine can inhibit Ca2+ current without changing [Ca2+]i. Single-channel recordings show that caffeine reduces mean open time without affecting single-channel conductance of L-type channels. Hence the effects of caffeine on ion channels in GH3 cells are attributable both to mobilization of intracellular Ca2+ and to a direct effect on the gating of L-type Ca2+ channels.

Buffering of calcium influx by sarcoplasmic reticulum during the action potential in guinea-pig ventricular myocytes

1. Intracellular [Ca2+] ([Ca2+]i) transients, monitored by the fluorescent Ca2+ indicator, indo-1, and twitch contractions elicited by action potentials, by voltage clamp pulses or by rapid, brief pulses of caffeine, were measured in guinea-pig single ventricular myocytes. Experiments were designed to determine whether and to what extent the trans-sarcolemmal Ca2+ influx is immediately sequestered by the sarcoplasmic reticulum (SR). 2. Rapid, brief (100-200 ms) pulses of caffeine onto a rested myocyte elicited a [Ca2+]i transient and a contraction. Following exposure to specific SR inhibitors, ryanodine (100 nM) or thapsigargin (200 nM), the rapid application of caffeine onto a rested myocyte failed to elicit changes in [Ca2+]i or in cell length, indicating that caffeine increases [Ca2+]i by specifically discharging Ca2+ from the SR. In the absence of these inhibitors, a second pulse of caffeine, within 3 min following a prior pulse, failed to elicit a [Ca2+]i transient or contraction, indicating that a caffeine pulse depletes the SR releasable Ca2+ pool. 3. Following Ca2+ depletion of the SR by double caffeine pulses at rest, an electrical stimulation elicited a slow increase in [Ca2+]i, and, after a delay, a small, slow twitch contraction. The simultaneous application of caffeine and electrical stimulation of cells in which the SR was Ca2+ depleted elicited [Ca2+]i transients with an increased rate of rise and a larger amplitude (53 +/- 8 and 63 +/- 9% respectively; mean +/- S.E.M., n = 21) than those elicited by electrical stimulation alone. 4. Whether caffeine affected the L-type calcium current (ICa) elicited by electrical stimulation was determined under whole-cell voltage clamp. A caffeine pulse delivered at the onset of a depolarizing voltage clamp step also increased the rates of rise and the amplitudes of the [Ca2+]i transients and twitch contractions in cells in which the SR was depleted of Ca2+. However, Ca2+ influx via ICa decreased when caffeine was pulsed in conjunction with the voltage clamp, as the peak ICa was either unchanged or decreased while its inactivation was consistently accelerated. 5. Because the stimulation-dependent trans-sarcolemmal Ca2+ influx via ICa is not increased by a caffeine pulse, the augmentation of the rates of rise and the amplitudes of the electrically stimulated [Ca2+]i transients by caffeine pulsed in conjunction with the electrical stimulation in cells in which the SR had been depleted of Ca2+ indicates that a portion of Ca2+ influx during depolarization in the absence of caffeine is rapidly buffered by the SR.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of Ca2+ current in ileal cells by cyclopiazonic acid and ryanodine

The effects of cyclopiazonic acid and its combination with ryanodine on the inward Ca2+ current (ICa) were investigated in smooth muscle cells isolated from the circular layer of guinea-pig ileum. The ICa of these cells exhibited two components: a low-threshold, nicardipine (5 microM)-resistant, fast-inactivating component and a high-threshold, nicardipine-blockable and slowly inactivating component. Neither cyclopiazonic acid (up to 10 microM) nor ryanodine (10 microM) was able to affect both these components of ICa, when applied separately. Cyclopiazonic acid and ryanodine combination led to total abolishment of the high-threshold component, leaving the low-threshold component unaffected. The data presented suggest a process of Ca(2+)-dependent inactivation of the high-threshold component, elicited by an increase in the subplasmalemmal Ca2+ concentration due to Ca2+ released from the sarcoplasmic reticulum. It is considered that the combination of cyclopiazonic acid and ryanodine can be used as a valuable method to study the calcium sensitivity of both components of the ICa.

A possible role of sarcoplasmic Ca2+ release in modulating the slow Ca2+ current of skeletal muscle

Ca2+ channels are regulated in a variety of different ways, one of which is modulation by the Ca2+ ion itself. In skeletal muscle, Ca2+ release sites are presumably located in the vicinity of the dihydropyridine-sensitive Ca2+ channel. In this study, we have tried to investigate the effects of Ca2+ release from the sarcoplasmic reticulum on the L-type Ca2+ channel in frog skeletal muscle, using the double Vaseline gap technique. We found an increase in Ca2+ current amplitude on application of caffeine, a well-known potentiator of Ca2+ release. Addition of the fast Ca2+ buffer BAPTA to the intracellular solution led to a gradual decline in Ca2+ current amplitude and eventually caused complete inhibition. Similar observations were made when the muscle fibre was perfused internally with the Ca2+ release channel blocker ruthenium red. The time course of Ca2+ current decline followed closely the increase in ruthenium red concentration. This suggests that Ca2+ release from the sarcoplasmic reticulum is involved in the regulation of L-type Ca2+ channels in frog skeletal muscle.

Effects of excitatory neurotransmitters on Ca2+ channel current in smooth muscle cells isolated from guinea-pig urinary bladder

1. A whole-cell voltage clamp technique was used to examine the effects of purinoceptor and muscarinic receptor agonists on voltage-sensitive Ca2+ channels in guinea-pig isolated urinary bladder cells. 2. When the cell membrane was clamped at the holding potential, rapid application of ATP elicited a large inward current in normal solution containing 2.5 mM Ca2+, and reduced the subsequent Ca2+ channel current evoked by a depolarizing pulse (0 mV). Carbachol (CCh) elicited little membrane current, but similarly reduced the Ca2+ current. 3. When purinoceptor agonists were rapidly applied during conditioning depolarizations at +80 mV, an outward current was elicited, and the Ca2+ channel current evoked by the subsequent test potential of 0 mV was not affected. Application of CCh at +80 mV also elicited an outward current, but it reduced the subsequently evoked Ca2+ current. 4. The inhibitory effect of muscarinic agonists on the Ca2+ channel current was attenuated by caffeine (10 mM). 5. In Ca(2+)-free, low-Mg2+ solution, a Na+ current flowing through voltage-dependent Ca2+ channels was evoked by depolarization. This current was not reduced by bath application of purinoceptor agonists (ATP and alpha,beta-methylene ATP). 6. These results suggest that the main effect of purinoceptor stimulation is opening of non-selective cation channels, and that muscarinic stimulation triggers Ca2+ release from intracellular stores. Voltage-sensitive Ca2+ channels are inactivated through an increase in intracellular Ca2+ induced by either activation of purinoceptor or muscarinic receptors.

Inhibitory effects of histamine and bradykinin on calcium current in smooth muscle cells isolated from guinea-pig ileum

1. Single smooth muscle cells were isolated from the longitudinal muscle layer of the guinea-pig ileum and within 10 h Ca(2+)-currents (ICa) were recorded using the whole-cell patch clamp technique. 2. Histamine (10 microMs) and bradykinin (BK, 1 microM) suppressed ICa; the effect had two phases: a rapid and transient suppression of ICa followed by a sustained suppression. Acetylcholine and substance P appeared to have similar effects but these were not investigated in detail. 3. The effects of histamine and BK on ICa were established by high intracellular concentrations of the Ca2+ buffer EGTA (30 mM) or 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) (5 mM) in the absence of Ca2+ added to the pipette solution. When [Ca2+]i was strongly buffered to 125 or 190 nM by BAPTA-Ca2+ mixtures in the pipette the transient suppression of ICa was blocked but the sustained effect still occurred. This indicated that the transient effect was caused by a rise in [Ca2+]i. The sustained effect, in contrast, did not seem to be caused by a rise in [Ca2+]i but did show Ca2+ dependence because it did not occur if [Ca2+]i was abnormally low. 4. Application of caffeine (10 mM) to deplete stored Ca2+ or intracellular heparin (1 mM) to block the action of D-myo-inositol 1,4,5-trisphosphate (IP3) to release stored Ca2+ prevented the transient but not the sustained suppression of ICa. Heparin also blocked the transient Ca(2+)-activated K+ current in response to histamine or BK. Both transient and sustained suppressions of Ca2+ channel activity were observed in the absence of extracellular Ca2+ when current was carried mostly by Na+ ions. 5. Intracellular guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S; 10 or 100 microM) induced a gradual decline of ICa upon which transient decreases of current were superimposed. Histamine caused a larger than normal inhibition of ICa and no recovery occurred on wash-out. Intracellular guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S; 1 mM) abolished the effects of histamine and BK on ICa.(ABSTRACT TRUNCATED AT 400 WORDS)