Abstract
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 ET a antagonists. ET s increased the amplitude of intracellularCa 2+ transient (CaT) in indo-1 loaded rabbit ventricular myocytes, but the increase was much smaller than that produced by elevation of [Ca 2+] o or isoproterenol for a given extent of PIE, an indication of increased myofibrillar Ca 2+ sensitivity. ET s 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 Ca 2+ 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 ET s 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 +-Ca 2+ exchanger, and the latter, to an increase in myofibrillar Ca 2+ 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 Ca 2+ 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 Ca 2+ channels may play a crucial role in the cardiac action of ET isopeptides in the rabbit ventricular myocardium.
Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
