Vol. 298, Issue 3, 954-963, September 2001

In Contrast to Forskolin and 3-Isobutyl-1-methylxanthine, Amrinone Stimulates the Cardiac Voltage-Sensitive Release Mechanism without Increasing Calcium-Induced Calcium Release

Wei Xiong, Heidi M. Moore, Susan E. Howlett and Gregory R. Ferrier

Department of Pharmacology, Dalhousie University, Halifax, Nova Scotia, Canada

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.