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

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 (ICa(L)) and contractions by muscarinic agonists.

Voltage clamp pulses elicited ICa(L) that evoked contractions recorded with an edge detector in single guinea pig ventricular myocytes superfused with Tyrode's solution (36 degrees C). InsP3 or cyclic AMP (cAMP) was dialyzed into the cell at selected times via the patch electrode.

InsP3 (1-10 microM) transiently increased isotonic contractions when applied for 4-5 min; higher concentrations (50-300 microM) caused a sustained decrease in contractions. InsP3 had no effect on ICa(L) at any concentration tested. Caffeine (10 mM)-induced contractures were increased and decreased, respectively, at 3 and 100 microM InsP3. Pentosan polysulfate (50 micrograms/ml), an InsP3 receptor antagonist, opposed the increased contractions by InsP3. Intrapipette cyclic AMP (10-300 microM) caused sustained increases of ICa(L) and contractions. Cyclic AMP, but not InsP3, also increased ICa(L) when intrapipette Cs+ suppressed K+ currents.

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 InsP in the persistent increase of contractions by muscarinic agonist in ventricular muscle and myocytes. The failure of InsP3 to change ICa(L) when contractions were increased or decreased militates against the L-type calcium channel being an effector of InsP3.