Abstract

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°C. Na⁺ current was blocked. We compared effects of agents that elevate cAMP through activation of adenylyl cyclase (1 μM forskolin), nonspecific inhibition of phosphodiesterases (PDEs) [100 μM 3-isobutyl-1-methylxanthine (IBMX)], and selective inhibition of PDE III (100–500 μM amrinone) on contractions initiated by the VSRM and CICR. Forskolin and IBMX 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.