Mechanism of slow discharges of sheep carotid artery.

1. Single and double sucrose-gap methods were used to follow changes in membrane potential and conductance of smooth muscle of sheep carotid arteries. 2. K depolarization induced discharges lasting several seconds in various solutions containing Mn, or Mg, or Ca and procaine, sometimes with no other added cations and with only sulphate or ethanesulphonate as anions. 3. Membrane conductance usually rose substantially above resting level in the early part of these discharges, but fell towards resting level during the later part. 4. Large depolarizing currents caused porportionately less voltage displacement than small currents, and reduced voltage displacements induced by superimposed current pulses, even in Cl and HCO3 free solutions, indicating activation of K conductance by depolarization. 5. When procaine was added, or Ca replaced by Mn or Mg, conductance was lower both at rest and on depolarization, and the increase on depolarization often underwent slow inactivation which it never did in simple Ca containing solutions. 6. The results indicate that a slowly inactivated inward current dependent on Ca, Mn or Mg was largely responsible for the slow discharges, and that procaine, Mn or Mg assisted the discharges by reducing the normal rapid outward-rectifying K conductance and allowing it to inactivate on prolonged depolarization.