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 ET(A2) subtype that is less sensitive to BQ123 and FR139317, whereas the PIE of ET-3 is mediated by an ET(A1) 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-Nisopropyl) 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.