The activation of calcium and calcium‐activated potassium channels in mammalian colonic smooth muscle by substance P.

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Abstract
The regulation of Ca2+-activated K+ channels by the agonist substance P in freshly dissociated smooth muscle cells from the rabbit longitudinal colonic muscle was characterized using the patch clamp technique. In the cell-attached recording mode, when pipette and bath solutions contained equal [K+] (126 mM), the Ca2+-activated K+ channels showed a linear current-voltage relationship (between - 50 mV and 50 mV) with a slope conductance of 210 .+-. 35 pS (n = 12). Reversal potential measurements indicated that the channel was slightly selective for K+ over Na+ (PK/PNa = 110). Channels were activated by depolarizing membrane voltages and cytosolic Ca2+, and in inside-out patches channel activation depended sigmoidally on voltage and [Ca2+]. The potential for half-activation at a cytosolic [Ca2+] of 5 .times. 10-6 M was 0 mV. A tenfold increase in cytosolic Ca2+ resulted in a 60 mV shift of the sigmoidal voltage activation curve to more negative potentials. Threshold concentrations of substance P (10-12 M), which did not result in cell contraction, caused a prolonged activation of K+ channels. The K+ channels were observed to open in clusters: simultaneous opening of multiple channels was interrupted by complete, prolonged channel closure. Lowering bath [Ca2+] to submicromolar concentrations abolished the effect of substance P. The activation of K+ channels by substance P (10-12 M) was also inhibited by the dihydropyridine nifedipine(10-6 M), a blocker of L-type Ca2+ channels. In the whole-cell recording mode, with the pipette solution containing 126 mM-KCl, 0.77 mM-EGTA and 1 mM-ATP, depolarization from a holding potential of -70 mV elicited outward currents which increased to steady-state values. These were K+ currents as they were blocked by TEA (tetraethylammonium, 30 mM) and Ba2+ (1 mM) and were abolished when pipette K+ was replaced by Cs+. The depolarization-activated outward current was not affected by lowering extracellular [Ca2+] or by the Ca2+ channel antagonists Cd2+ (200 .mu.M), nifedipine (10-6-10-5 M) or verapamil (10-6 M). The current was greatly reduced when the EGTA concentration in the pipette solution was increased from 0.77 to 10 mM. When the pipette solution contained CsCl, membrane depolarization activated inward currents. The peak inward current was identified as current through L-type Ca2+ channels based on its voltage- and time-dependent kinetics, and its modulation by dihydropyridines. The NK-1 receptor agonist substance P methyl ester (SPME) increased dose dependently (10-12-10-10 M) the amplitude and the rate of rise of the depolarization-activated outward current at membrane potentials between -20 mV and 50 mV. This increase was abolished by lowering bath [Ca2+], and by adding Cd2+ (200 .mu.M) or nifedipine (10-6 M) to the bath. When the pipette was filled with 140 mM-CsCl, substance P methyl ester (10-12-10-10 M) increased dose dependently the amplitude and the rate of decline of the voltage-activated Ca2+ current. No inward current was seen in the presence of nifedipine (10-6 M). These results indicate that substance P in the concentration range 10-12-10-10 M increases the open probability of Ca2+-activated K+ channels. This effect is predominantly mediated by the activation of L-type Ca2+ channels via interaction with a high-affinity neurokinin-1 (NK-1) receptor.