ENDOTHELIN-1 DOES NOT ALTER CA2-SKINNED FERRET PAPILLARY-MUSCLES( RESPONSIVENESS IN SAPONIN)

Endothelin is a potent vasoconstrictor and a positive inotropic agent in myocardium. Endothelin has been reported to increase myocardial con tractility with little or no increase in intracellular Ca2+, thus appa rently enhancing myofilament responsiveness to Ca2+. We investigated t he effects of endothelin on tension development and Ca2+ responsivenes s in both intact and saponin-skinned ferret right ventricular papillar y muscles. Isolated ferret papillary muscles were stimulated for 2 h i n the presence or absence of endothelin (100 nM). The muscles were the n chemically skinned with saponin and exposed to relaxing and contract ing solutions containing varying amounts of Ca2+, and the developed fo rce of contraction was measured. The [Ca2+] required for half-maximal activation (pCa(50)) was determined by fitting force versus Ca2+ data to the Hill equation. In isometrically contracting muscles, endothelin (100 nM) caused a mean percent increase in developed tension of 34.7% +/- 11.3% (mean +/- S.E.). In muscles that were exposed to endothelin for 2 h and then skinned, neither the pCa(50) nor the maximal Ca2+-ac tivated force (F-max) were significantly different from control skinne d papillary muscles. After skinning, when endothelin (100 nM) was adde d to the Ca2+ buffers, both pCa(50) and F-max were significantly decre ased. When papillary muscles were pretreated with phorbol 12-myristate 13-acetate (PMA) and then skinned, there was a significant increase i n the pCa(50). These results indicate that endothelin acts directly on the myofilaments to impair force development by directly decreasing t he Ca2+ responsiveness of myofilaments. Therefore, the positive inotro pic effect and the increased Ca2+ responsiveness produced by endotheli n in intact muscles is probably mediated by a receptor-second messenge r system which is inactivated by the skinning process.