Endothelin: receptor subtypes, signal transduction, regulation of Ca2+ transients and contractility in rabbit ventricular myocardium.
Endothelin (ET) isopeptides, ET-1, ET-2 and ET-3, elicit a positive inotropic effect (PIE) in association with a negative lusitropic effect, essentially with identical efficacies and potencies in the isolated rabbit papillary muscle, but with different concentration-dependent properties. Pharmacological analysis indicates that the PIE of ET-1 is mediated by an ETA2 subtype that is less sensitive to BQ-123 and FR139317, whereas the PIE of ET-3 is mediated by an ETA1 subtype that is highly sensitive to these ETA antagonists. ETs increased the amplitude of intracellular Ca2+ transient (CaT) in indo-1 loaded rabbit ventricular myocytes, but the increase was much smaller than that produced by elevation of [Ca2+]o or isoproterenol for a given extent of PIE, an indication of increased myofibrillar Ca2+ sensitivity. ETs stimulate phosphoinositide (PI) hydrolysis, which leads to production of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). Evidence for the role of IP3-induced Ca2+ release in cardiac E-C coupling is tenuous. Generation of IP3 induced by ET-1 was transient and returned to the baseline level when the PIE reached an elevated steady level. Protein kinase C (PKC) that is activated by DAG and also via other pathways triggered by ETs stimulates Na+-H+ exchanger to lead to an increased [Na+]i and alkalinization. The former may contribute to an increase in the amplitude of CaT through Na+-Ca2+ exchanger, and the latter, to an increase in myofibrillar Ca2+ sensitivity. A number of PKC inhibitors, such as staurosporine, H-7, calphostin C and chelerythrine, consistently and selectively inhibited the PIE of ET-3 without affecting the PIE of isoproterenol and Bay k 8644. The maximum inhibition was 20-30% of the total response. A Na+-H+ exchange inhibitor, [5-(N-ethyl-N-isopropyl) amiloride (EIPA)] or a Ca2+ antagonist, verapamil, could not completely inhibit the PIE of ET-3, but the combination of both inhibitors totally abolished the PIE of ET-3. These findings indicate that activation of PKC and subsequent activation of Na+-H+ exchanger and/or L-type Ca2+ channels may play a crucial role in the cardiac action of ET isopeptides in the rabbit ventricular myocardium.
Endoh
M
M
Department of Pharmacology, Yamagata University School of Medicine, Japan.
Fujita
S
S
Yang
H T
HT
Talukder
M A
MA
Maruya
J
J
Norota
I
I
D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D016232:Endothelins / Q000502:physiology; D006321:Heart / Q000502:physiology*; D006352:Heart Ventricles / Q000648:ultrastructure; D009200:Myocardial Contraction / Q000502:physiology*; D009206:Myocardium / Q000378:metabolism / Q000648:ultrastructure*; D011817:Rabbits; D017466:Receptors, Endothelin / Q000145:classification* / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology*
D016232:Endothelins; D017466:Receptors, Endothelin; D002118:Calcium