Alpha-adrenoceptor-mediated restoration of calcium-dependent potential in the partially depolarized rabbit papillary muscle

Summary

Electrophysiological and mechanical effects of phenylephrine were observed in the rabbit papillary muscle which was depolarized and rendered inexcitable by an elevation of potassium concentration to 27 mM in the presence of a β-adrenoceptor blocking drug, bufetolol (10⁻⁶ M).

• 1. Phenylephrine (10⁻⁶ to 10⁻⁴ M) restored the action potential and tension development. The amplitude, duration and maximum rate of rise of the action potential, and developed tension were augmented by an increase in the concentration of phenylephrine.

• 2. The amplitude of the action potential restored by phenylephrine was very sensitive to the Ca²⁺ concentration and increased by 29.2 mV/10-fold increase crease in external Ca²⁺ concentration. Nifedipine (10⁻⁶ M), a Ca²⁺ antagonist, completely suppressed the electrical and mechanical responses restored by phenylephrine (10⁻⁵ M), but tetrodotoxin (10⁻⁵ M) had little effect on these responses.

• 3. Phentolamine (10⁻⁶ M) suppressed both the action potential and developed tension restored by phenylephrine (10⁻⁵ M), but the elevation of the concentration of bufetolol from 10⁻⁶ M to 3×10⁻⁶ M was ineffective against these responses.

In conclusion, the restoration by phenylephrine of Ca²⁺-dependent action potential may be ascribed to an enhancement of the inward Ca²⁺ current, mediated by α-adrenoceptors and this mechanism may play an important role in the α-adrenoceptor-mediated positive inotropic effect.