Thrombin stimulates Ca-channel currents in isolated frog ventricular cells

Thrombin receptor actions in neonatal rat ventricular myocytes

Previous studies established that thrombin stimulates phosphoinositide hydrolysis and modulates contractile function in neonatal rat ventricular myocytes. The present study further defines the signaling pathways activated by the thrombin receptor and their role in thrombin's actions in cardiac myocytes. The thrombin receptor-derived agonist peptide (TRAP, a portion of the tethered ligand created by thrombin's proteolytic activity) stimulates the rapid and transient accumulation of inositol bis- and tris-phosphates (IP2 and IP3, respectively), which is followed by the more gradual and sustained accumulation of inositol monophosphate (IP1). TRAP elicits a larger and more sustained accumulation of IP1 than does thrombin. Thrombin and TRAP also activate mitogen-activated protein kinase (MAPK) in cultured neonatal rat ventricular myocytes. Differences in the kinetics and magnitude of thrombin- and TRAP-dependent inositol phosphate (IP) accumulation are paralleled by differences in the kinetics and magnitude of thrombin- and TRAP-dependent activation of MAPK. Pretreatment with phorbol 12-myristate 13-acetate (PMA) to downregulate protein kinase C (PKC) attenuates thrombin- and TRAP-dependent activation of MAPK, although small and equivalent effects of thrombin and TRAP to stimulate MAPK persist in PMA-pretreated cells. These results support the notion that the thrombin receptor activates MAPK through PKC-dependent pathways and that the incremental activation of MAPK by TRAP over that induced by thrombin is the consequence of enhanced activation through the PKC limb of the phosphoinositide lipid pathway. TRAP also increases the beating rate of spontaneously contracting ventricular myocytes and elevates cytosolic calcium in myocytes electrically driven at a constant basic cycle length. The effects of TRAP to modulate contractile function and elevate intracellular calcium are not inhibited by tricyclodecan-9-yl-xanthogenate (D609, to block TRAP-dependent IP accumulation) or pretreatment with PMA (to downregulate PKC). The TRAP-dependent rise in intracellular calcium also is not inhibited by verapamil or removal of extracellular calcium but is markedly attenuated by depletion of sarcoplasmic reticular calcium stores by caffeine. Patch-clamp experiments demonstrate that TRAP elevates intracellular calcium in cells held at a membrane potential of -70 mV. Taken together, these results support the conclusion that the thrombin receptor modulates contractile function by mobilizing intracellular calcium through an IP3-independent mechanism and that this response does not require activation of voltage-gated ion channels.

Dual effect of thrombin on voltage-dependent Ca2+ channels of portal vein smooth muscle cells

Thrombin induces a number of physiological responses in several types of cells. To determine the action of thrombin in the vein, the electrophysiological and mechanical effects of thrombin were studied in rat portal vein smooth muscle cells. Ca2+ channel currents were recorded using the whole-cell patch-clamp technique. Thrombin had both inhibitory and stimulatory effects on the Ca2+ channel current. The inhibitory effect was reversed on washout of thrombin, whereas the stimulatory effect was maintained after thrombin was removed. Thrombin (1 unit/ml) produced a reversible decrease of 27.3 +/- 3.3% (n = 12) in the current amplitude and a sustained increase of 71.2 +/- 12.9% (n = 20). The thrombin-induced inhibition of Ca2+ channel current was blocked by the thrombin inhibitor hirudin and by the protease inhibitor leupeptin. The stimulatory effect of thrombin was inhibited by hirudin, by intracellular application of guanosine 5'-O-(beta-thio)diphosphate, and by antiphophatidylinositide antibodies but not by pertussis toxin. The thrombin-induced enhancement of the Ca2+ channel current amplitude was not observed when the current was previously stimulated by phorbol 12,13-dibutyrate. This suggests that the inhibitory effect of thrombin was related to its proteolytic activity and that the stimulatory effect involved activation of a pertussis toxin-insensitive GTP-binding protein, phosphatidylinositide hydrolysis, and protein kinase C activation. Both thrombin effects occurred in the same concentration range (0.001-10 units/ml). The thrombin-induced contraction of portal vein strips was completely inhibited by isradipine, and thrombin did not produce an increase in cytosolic [Ca2+], measured by indo-1 fluorescence in cells clamped at -50 mV, sufficient to activate Ca(2+)-dependent chloride current.(ABSTRACT TRUNCATED AT 250 WORDS)

Thrombin stimulates L-type calcium channels of guinea pig cardiomyocytes in cell-attached patches but not after intracellular dialysis

The action of the blood clotting enzyme thrombin on single channel and whole cell Ca(2+)-currents was studied in isolated mammalian cardiac myocytes. Thrombin, at a concentration of 10(-8) mol/l, increased the Ca(2+)-channel activity in cell-attached patches. The mean open probability of the channel was enhanced, while the number of sweeps without openings, which reflects the availability of the channel, was significantly reduced. Neither the single channel conductance nor the activation curve were affected by thrombin. Thrombin was added to the bath solution, and its effect is therefore indirect and probably mediated via a second messenger. However, thrombin did not affect whole-cell Ca(2+)-currents, whereas a beta-adrenergic stimulation in the same cell increased the Ca(2+)-current. It is concluded that thrombin affects an intracellular mechanism for Ca2+ channel current regulation, which is still unknown and which is rapidly lost during conventional whole-cell Ca2+ current measurements.

Thrombin modulates phosphoinositide metabolism, cytosolic calcium, and impulse initiation in the heart

Thrombin stimulates phosphoinositide hydrolysis and increases cytosolic calcium in several types of cells. To determine whether thrombin exerts similar stimulatory actions in the heart and whether this mechanism is linked to changes in cardiac electrical activity, the effects of thrombin on several biochemical and electrophysiological parameters were examined. In neonatal rat ventricular myocyte cultures freed of fibroblast contamination by irradiation, thrombin rapidly induced the breakdown of phosphoinositides. Formation of inositol trisphosphate was detectable within 5 seconds and was followed by the sequential accumulation of inositol bisphosphate and inositol monophosphate. The effect of thrombin to stimulate phosphoinositide hydrolysis was inhibited by hirudin, but not by propranolol, prazosin, or pretreatment with pertussis toxin. The inositol phospholipid response was unassociated with changes in intracellular cAMP levels. To determine the electrophysiological effects of thrombin, we used microelectrode techniques to study canine Purkinje fibers. Thrombin increased the beating rate of fibers depolarized using barium, but not those at normal maximal diastolic potential. In addition, thrombin prolonged the action potential duration in fibers driven at a constant cycle length. This response was inhibited by hirudin and nisoldipine, but not by propranolol, prazosin, or pretreatment with pertussis toxin. Thrombin also augmented cesium-induced early afterdepolarizations. Using the fluorescent calcium indicator fura-2, we demonstrated that thrombin increased the beating rate, diastolic calcium, and peak systolic calcium of spontaneously contracting cultured ventricular myocytes. Cytosolic calcium also increased in both rat ventricular myocytes and canine Purkinje myocytes that were electrically driven at a constant basic cycle length, indicating that thrombin modulates cellular calcium metabolism independent of its actions to enhance automaticity. Taken together, these findings demonstrate several novel biological actions of thrombin in the mammalian heart that may be functionally related. The actions of thrombin to enhance automaticity and prolong repolarization may contribute to the electrical abnormalities observed in the setting of myocardial ischemia and infarction.

Effect of thrombin on calcium homeostasis in chick embryonic heart cells. Receptor-operated calcium entry with inositol trisphosphate and a pertussis toxin-sensitive G protein as second messengers

Thrombin increases intracellular calcium ([Ca++]i) in several cell types and causes a positive inotropic effect in the heart. We examined the mechanism of the thrombin-induced [Ca++]i increase in chick embryonic heart cells loaded with the fluorescent calcium indicator, indo-1. Thrombin (1 U/ml) increased both systolic and diastolic [Ca++]i from 617 +/- 62 and 324 +/- 46 to 1041 +/- 93 and 587 +/- 38 nM, respectively. An initial rapid [Ca++]i increase was followed by a more sustained increase. There were associated increases in contraction strength, beat frequency, and action potential duration. The [Ca++]i increase was not blocked by tetrodotoxin or verapamil, but was blocked by pretreatment with pertussis toxin (100 ng/ml). The thrombin-induced [Ca++]i increase was partly due to intracellular calcium release, since it persisted after removal of external calcium. The [Ca++]i increase in zero calcium was more transitory than in normal calcium and was potentiated by 10 mM Li+. Thrombin also induced influx of calcium across the surface membrane, which could be monitored using Mn++ ions, which quench indo-1 fluorescence when they enter the cell. Thrombin-induced Mn++ entry was insensitive to verapamil, but was blocked by 2 mM Ni++. Thrombin increased inositol trisphosphates by 180% at 90 s and this effect was also blocked by pretreatment with pertussis toxin.

CONCLUSION

thrombin promotes calcium entry and release in embryonic heart cells even when action potentials are inhibited. Both modes of [Ca++]i increase may be coupled to the receptor by pertussis toxin-sensitive G proteins.

Pharmacological modification of mechanical and electrical responses of frog heart to thrombin

The pharmacological modification of the thrombin effect on the mechanical and electrical responses of frog heart was examined in the Straub heart preparation and in single ventricular cells. Associated with the positive inotropic action thrombin increases voltage and duration of action potentials of isolated frog ventricular cells. As found by the patch-clamp technique in the cell-attached mode, thrombin stimulates single L-type Ca2+ channels, presumably mediated by a second messenger. The enhancement of contractility by thrombin depends on the proteolytic activity of the enzyme because enzymatically inactivated thrombin has no effect on frog hearts. The positive inotropic effect of thrombin as well as its stimulation of Ca2+ channel currents were inhibited by the protein kinase C blocker 1-(5-isoquinoline-sulfonyl)-2-methylpiperazine (H7). However, phorbol 12-myristate 13-acetate (PMA), a known stimulator of protein kinase C, was ineffective in stimulating the inotropic action of thrombin. The inhibition of the thrombin-induced enhancement of contractility by indometacin indicates an involvement of arachidonic acid in the action of thrombin on frog heart.

Effects of thrombin on single calcium channels in frog ventricular cells

Single calcium channel (Ca channel) currents were measured using the patch-clamp technique in isolated ventricular myocytes of the frog (Rana esculenta). Sodium was used as the charge carrier. After formation of cell-attached patches, the proteolytic enzyme thrombin was added to the bath solution, where it increased the amplitude of the averaged currents more than twofold, by decreasing the number of empty sweeps and reducing the time constant of the slow exponential term of the shut-time histogram. Single channel conductance was not changed by thrombin. If the activation kinetics of the Ca channels are described by the commonly used C1-C2-O model, where C1 and C2 indicate closed states 1 and 2 respectively and O denotes the open state, thrombin increases the open-state probability in the non-empty sweeps by increasing the rate constant (k1) for the transition from C1 to C2. It is shown that thrombin acts via an H-7 blockable pathway.

An endogenous 'hypertensive factor' enhances the voltage-dependent calcium current

The effects of an immunoaffinity-purified putative endogenous hypertensive factor (HF) on voltage-dependent calcium current in frog cardiac myocytes were assessed. In 9 out of 10 cells, HF reversibly increased the peak amplitude of the calcium current. HF increased peak calcium current density at -5 mV from a control level of 1.8 +/- 1.3 pA/pF (mean +/- SD) to 4.4 +/- 2.0 pA/pF. HF shifted the peak of the calcium current-voltage relationship in the hyperpolarizing direction. HF shifted the voltage dependence of the inactivation of the calcium current to more negative potentials with prepulses from -80 to 0 mV, but the inactivation was not affected with prepulses more positive than 0 mV. Modulation of the voltage-dependent calcium current by HF may be the mechanism underlying its pressor effects.