Inositol-1,4,5-trisphosphate increases contractions but not L-type calcium current in guinea pig ventricular myocytes

Objective: We studied the effects of intracellularly applied inositol-1,4,5-trisphosphate (InsP3) to test the hypothesis that InsP3 is a messenger for stimulation of L-type calcium current ( I Ca(L)) and contractions by muscarinic agonists. Methods: Voltage clamp pulses elicited I Ca(L) that evoked contractions recorded with an edge detector in single guinea pig ventricular myocytes superfused with Tyrode’s solution (36°C). InsP3 or cyclic AMP (cAMP) was dialyzed into the cell at selected times via the patch electrode. Results: InsP3 (1–10 μM) transiently increased isotonic contractions when applied for 4–5 min; higher concentrations (50–300 μM) caused a sustained decrease in contractions. InsP3 had no effect on I Ca(L) at any concentration tested. Caffeine (10 mM)-induced contractures were increased and decreased, respectively, at 3 and 100 μM InsP3. Pentosan polysulfate (50 μg/ml), an InsP3 receptor antagonist, opposed the increased contractions by InsP3. Intrapipette cyclic AMP (10–300 μM) caused sustained increases of I Ca(L) and contractions. Cyclic AMP, but not InsP3, also increased I Ca(L) when intrapipette Cs+ suppressed K+ currents. Conclusions: Increased myocyte shortening at low InsP3 concentrations accords with receptor-initiated sarcoplasmic reticulum Ca2+ release. The transient stimulation of contractions at low concentrations and the sustained reduction of contractions at high concentrations are not consistent with a role for InsP3 in the persistent increase of contractions by muscarinic agonist in ventricular muscle and myocytes. The failure of InsP3 to change I Ca(L) when contractions were increased or decreased militates against the L-type calcium channel being an effector of InsP3.