Cellular actions of halothane on cat cerebral arterial muscle.

The effects of halothane on intracellular membrane potential (Em) and force development in cat MCA were studied. Halothane (0.07-0.14 mM/1) relaxed isolated MCA which had developed myogenic tone. Measurement of Em showed that halothane depolarized this preparation in a dose-dependent fashion in the face of vessel relaxation, demonstrating uncoupling of electrical and mechanical activity. Halothane markedly inhibited the contractile effects of histamine and serotonin suggesting that, apart from its direct action on cerebral arterial tone, it also blunts the action of vasoactive agents. When this preparation is partially depolarized from -62 to -50 mV with excess K+, halothane, while having only a small (1.2 mV) additional depolarizing effect, consistently elicits contraction rather than relaxation. Thus, the action of this particular volatile anesthetic on cerebral arteries can depend upon the resting level of Em. These studies indicate that halothane relaxes myogenic tone in cat MCA by an intracellular mechanism, but that the direction of its effect (i.e., relaxation vs. contraction) may depend upon the prior level of Em and muscle cell activation.