Upstroke component of electrical slow waves in canine colonic smooth muscle due to nifedipine‐resistant calcium current.

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Abstract
1. Electrical slow waves of gastrointestinal smooth muscles are not abolished by organic Ca2+ channel blocking drugs, such as nifedipine or D600. These compounds reduce the amplitude and duration of the plateau phase, but the upstroke phase of slow waves persists. 2. Voltage clamp experiments were performed on isolated circular muscle cells from the canine proximal colon to characterize the dihydropyridine-resistant component of inward current. Inward currents were measured at 25 and 35 degrees C. The higher temperature increased the amplitudes of the transient and sustained phases of the inward current. The voltage dependence of activation and inactivation of the inward current was not significantly changed at 35 vs. 25 degrees C. 3. At 35 degrees C the transient phase of the inward current was reduced but not blocked by nifedipine (10(-6) M). The sustained phase was blocked by nifedipine. 4. The block by nifedipine was voltage dependent, increasing with depolarization. At voltages reached during the upstroke depolarization about 35% of the inward current persisted in the presence of nifedipine (10(-6) M). This may be sufficient inward current to sustain the upstroke depolarization in intact muscles. 5. Nifedipine caused a 20 mV negative shift in the voltage dependence of inactivation suggesting that dihydropyridines may preferentially bind to Ca2+ channels in an inactivated state. 6. Ni2+ (< 100 microM) significantly decreased the transient phase of inward current. A combination of Ni2+ (40 microM) and nifedipine (10(-6) M) blocked all of the inward current at 35 degrees C. Combination of nifedipine (10(-6) M) and Ni2+ (40 microM) blocked slow waves in intact muscles. 7. Bay K 8644 (10(-6) M) increased the amplitude of the transient and sustained components of inward current. On a percentage basis the increase in the sustained component was greater than the increase in the transient component with test potentials in the range of -50 to -20 mV. This may explain why Bay K 8644 preferentially increases the plateau component of slow waves vs. the upstroke component. 8. The findings of this study suggest that the nifedipine resistance of the upstroke depolarization could be due to the voltage dependence of the block of Ca2+ channels by dihydropyridines. Thus a single class of voltage-dependent Ca2+ channels could be responsible for the upstroke and plateau phases of slow waves.