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Evidence for two mechanisms of depolarization associated with α1‐adrenoceptor activation in the rat anococcygeus muscle

1 Membrane potential responses in the rat isolated anococcygeus to bath‐applied noradrenaline and field stimulation have been investigated by use of intracellular microelectrode and combined extracellular electrical and mechanical (sucrose gap) recording techniques. Intracellular recordings were made usually from tissues immobilized with hypertonic Krebs solution. 2 Bath‐application of noradrenaline produced depolarizations which consisted of two components; an initial ‘fast’ phase which peaked within 1–2 s and which was followed by a ‘slow’ sustained depolarization. Both components were concentration‐dependent. 3 Noradrenaline could also evoke oscillations in membrane potential which, unlike the ‘fast’ component of depolarization, were prevented by conditioning hyperpolarization of the membrane and were evoked by direct membrane depolarization with externally applied current pulses. Thus, the oscillations are voltage‐dependent phenomena. 4 Replacement of the external NaCl of the Krebs solution with an equimolar amount of Na benzenesulphonate abolished the noradrenaline‐evoked ‘fast’ depolarization while the ‘slow’ phase was unaffected. This suggests that two mechanisms of depolarization are activated in this muscle by the bath‐application of noradrenaline. The adrenergic excitatory junction potential was also abolished in Na benzenesulphonate. 5 Prazosin reduced both the ‘fast’ and ‘slow’ components of depolarization produced by noradrenaline indicating their mediation by α1‐adrenoceptors. 6 The membrane potential (‐ 29 mV) at the maximum amplitude of the ‘fast’ depolarization was similar to the equilibrium potential (‐ 27 mV) for the depolarization evoked by ionophoretically applied noradrenaline and which was obtained by extrapolation from the relationship between amplitude of the ionophoretic response and membrane potential displacement in the partition chamber. These results suggest that the ‘fast’ depolarization and the ionophoretic response are due to an increased membrane conductance, possibly to chloride.