Effects of cAMP and forskolin on caffeine-induced contractures and myofilament Ca-sensitivity in saponin-treated rat ventricular trabeculae.

Trabeculae from the right ventricle of rat were chemically 'skinned' with saponin (50 micrograms ml-1 for 30 min). Caffeine (10 mM) induced a transient contracture as a consequence of Ca(2+)-release from the sarcoplasmic reticulum and subsequent activation of the myofilaments. The amplitude of the caffeine contracture was used as an index of the Ca(2+)-content of the sarcoplasmic reticulum. cAMP (0.1-10 microM) markedly potentiated the caffeine-induced response under standardized conditions. This was interpreted as a cAMP-induced increase in Ca2+ accumulation by the sarcoplasmic reticulum during the Ca2+ loading period before the application of caffeine. After removal of cAMP, the amplitude of the regularly evoked contractures slowly declined towards standard levels. The characteristic effects of cAMP on the caffeine contracture were mimicked by addition of forskolin (10(-6) M), a substance known to stimulate cAMP production by adenylate cyclase. The results suggest that a significant quantity of functional adenylate cyclase persists after saponin treatment. In these preparations cAMP had no effect on the apparent Ca(2+)-sensitivity of the myofilaments. If the preparations were exposed briefly to saponin, cAMP caused a decrease in the apparent Ca2+ sensitivity of the contractile proteins. However, further exposure to saponin, or to Triton X-100, caused a marked increase in maximum Ca(2+)-activated force (Cmax). It was concluded that 'briefly' saponin-treated preparations, exhibiting a reduction in Ca2+ sensitivity in response to cAMP, are not uniformly permeable to the bathing solution.