In contrast to forskolin and 3-isobutyl-1-methylxanthine, amrinone stimulates the cardiac voltage-sensitive release mechanism without increasing calcium-induced calcium release

The objective of this study was to determine whether the voltage-sensitive release mechanism (VSRM) can be stimulated independently from Ca2+-induced Ca2+ release (CICR) by drugs that elevate intracellular cAMP. Contractions were measured in voltage-clamped guinea pig ventricular myocytes at 37 degreesC. Na+ current was blocked. We compared effects of agents that elevate cAMP through activation of adenylyl cyclase (1 muM forskolin), nonspecific inhibition of phosphodiesterases (PDEs) [100 muM 3-isobutyl-1-methylxanthine (IBMX)], and selective inhibition of PDE III (100-500 muM amrinone) on contractions initiated by the VSRM and CICR. Forskolin and lBMX significantly increased peak Ca2+ current and CICR. In addition, these agents also markedly increased contractions elicited by test steps from -65 to -40 mV, which activate the VSRM. However, because these steps also induced inward current in the presence of forskolin or IBMX, CICR could not be excluded. In contrast, amrinone caused a large, concentration-dependent increase in VSRM contractions but had no effect on CICR contractions or Ca2+ current. Sarcoplasmic reticulum Ca2+, assessed by rapid application of caffeine (10 mM), was increased only modestly by all three drugs. Normalization of contractions to caffeine contractures indicated that amrinone increased fractional release by the VSRM, but not CICR. Forskolin and IBMX increased fractional release elicited by steps to -40 mV. Increases in CICR induced by forskolin and IBMX were proportional to caffeine contractures. Thus, positive inotropic effects of CAMP on VSRM contractions may be compartmentalized separately from effects on Ca2+ current and CICR.