GENERATION OF SLOW-WAVE-TYPE ACTION-POTENTIALS IN CANINE COLON SMOOTH-MUSCLE INVOLVES A NON-L-TYPE CA2+ CONDUCTANCE

1. The hypothesis was addressed that a non-L-type calcium conductance is involved in the generation of the initial part of the slow-wave-type action potential in the canine colon. 2. In the absence of a sodium and chloride gradient (NaCl replaced by glucamine), and in the presence of nitrendipine (in 'glucamine-nitrendipine' Krebs solution), a major portion of the upstroke potential of the slow wave persists at unchanged frequency. 3. In 'glucamine-nitrendipine' Krebs solution, the rate of rise and amplitude of the upstroke potential is reduced by removal of extracellular calcium in a concentration-dependent manner. 4. The rate of rise and the amplitude of the upstroke potential is in a concentration-dependent manner reduced by Ni2+ > Cd2+ > Co2+ > Mg2+. 5. In 'glucamine-nitrendipine' Krebs solution, Ba2+ cannot replace Ca2+ in the generation of the upstroke potential. 6. Positive evidence was obtained for the hypothesis that a non-L-type calcium conductance is involved in the initiation of the slow-wave-type action potential in colonic smooth muscle.