Effects of (−)-Phenylephrine on force of contraction in the presence of cocaine and hydrocortisone in cat papillary muscle

The positive inotropic effect of (−)-phenylephrine, in the presence of propranolol, was studied after inhibition of neuronal and extraneuronal uptake in isolated papillary muscles from reserpine-pretreated cats. An inhibition of extraneuronal uptake with hydrocortisone influenced the effect of (−)-phenylephrine neither when present alone nor in the presence of cocaine (additional inhibition of neuronal uptake). An inhibition of neuronal uptake with cocaine caused a small but significant increase in the potency of (−)-phenylephrine. We conclude that in cat ventricular cardiac muscle the extraneuronal compartment is apparently neither a “site of loss” nor a “site of gain” for (−)-phenylephrine with respect to force of contraction.