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

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.