In order to clarify the receptor types and mechanisms underlying the positive isotropic effect of phenylephrine on the mammalian ventricular myocardium, the action potential, its first derivatives and isometric contraction of the rabbit papillary muscle were recorded using a force transducer and glass capillary microelectrodes filled with 3M KCI, and results were analyzed.
The results were as follows;
1. In normal Tyrode solution, the contractile force was increased with increments of phenylephrine concentration and maximum effect was reached at the stimulation frequency of 1 Hz in 10´ M phenylephrine.
2. In 22 mM K+-Tyrode solution, the maximum rate of rise (Vmax) of action potential, overshoot and the duration of action potential were significantly increased with increments of phenylephrine concentration (10-6 ^-, 10-4 M).
3. The negative inotropic effect induced by verapamil was reversed by phenylephrine,
4. Phent olamine (3x10-b NI) shifted the dose-response curve for phenylephrine to the right more than propranolol (10-6 M) did.
The above results may be interpreted as follow;
The positive inotropic effect of phenylephrine is caused by increase in slow inward current (Ca 2+ influx into the `myocardial cell), and is mediated through a- and (i-adrengergic receptors in the rabbit paillary muscle.
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