Generation of slow‐wave‐type action potentials in canine colon smooth muscle involves a non‐L‐type Ca2+ conductance.

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Abstract
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+ greater than Cd2+ greater than Co2+ greater than 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.