The action potential and underlying ionic currents in proximal rat middle cerebral arterioles.

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Abstract
The active and passive electrical properties of isolated segments (length 120‐220 microns, diameter 60‐150 micron) of proximal rat middle cerebral arterioles (less than 1 mm from parent artery) were analysed using a single‐electrode current or voltage clamp. The voltage response to a current step exhibited an exponential time course. The mean resistance and time constant was 102 M omega and 265 ms corresponding to approximate specific resistance and capacitance of 60 k omega/cm and 4 micro F/cm2. Membrane resistance was constant in the range ‐55 to ‐80 mV. At potentials more negative than ‐80 mV there was a decrease in membrane resistance resulting in activation of an inward rectifier. At membrane potentials less negative than ‐50 mV the membrane resistance decreased; larger depolarizations (greater than ‐40 mV) initiated small regenerative responses. External application of tetraethylammonium chloride caused membrane depolarization (10‐15 mV), spontaneous discharge of action potentials and rhythmic arteriolar constriction. Action potentials studied with the membrane held at ‐60 mV had a large rapid depolarizing component, an after‐depolarization and a small slower after‐hyperpolarization. Tetrodotoxin (TTX) had no effect on the action potential. However, both the fast and slow components of the action potential were suppressed by extracellular removal of calcium ions and/or addition of cobalt ions, nifedipine or verapamil. Voltage‐clamp studies demonstrated an inward rectifying current at membrane potentials more negative than ‐80 mV. At depolarized potentials at least four separate currents were activated; two separate calcium currents and two outward currents.