Ion channel modulation by NS 1619, the putative BKCa channel opener, in vascular smooth muscle

Abstract

1 The effects of NS 1619, the putative BK_Ca channel opener, were investigated on rat intact portal veins and on single smooth muscle cells enzymatically separated from the same tissue. 2 Under whole-cell patch clamp conditions with K-rich pipettes, exposure of single cells held at −10 mV to NS 1619 (10–33 μm) induced a noisy, outward current which reached a maximum (33 μm NS 1619; mean 35.8 ± 17 pA, n = 8) within about 6min. 3 On stepping to test potentials (range − 50 to + 50 mV) from a holding potential of − 10 mV, the NS 1619-induced noisy current exhibited time-dependent activation and marked outward rectification. 4 The stimulation of outward currents by NS 1619 at − 10 mV was independent of the presence of Ca²⁺ in the bath or pipette solutions but was antagonized by either charybdotoxin (250 nm) or penitrem A (100 nm) in the bath solution. 5 Stationary fluctuation analysis of the noisy current induced by NS 1619 at − 10 mV yielded a value of 70 ± 8 pS (n = 4) (under the quasi-physiological conditions of the experiment) for the unitary conductance of the channel involved. 6 At − 10 mV, NS 1619 (10–33 μm) rapidly inhibited spontaneous transient outward currents. 7 With a holding potential of − 90 mV, NS 1619 (10–33 μm) produced a reduction of outward currents evoked by depolarizing steps to + 50 mV, an effect associated with marked inhibition of the delayed rectifier current, I_K(V). 8 NS 1619 (3–100 μm) produced a concentration-dependent inhibition of spontaneous activity in rat portal vein characterized by a reduction in the amplitude and duration of the tension waves. This inhibition was slightly potentiated in the presence of either charybdotoxin (250 nm) or penitrem A (1 μm). NS 1619 also totally inhibited contractions of rat aorta induced by KC1 (both 20 mm and 80 μm). 9 Under whole-cell recording conditions and using Cs-rich pipettes, Ca-currents evoked in portal vein cells by stepping from a holding potential of − 90 mV to test potentials in the range − 30 to + 50 mV were totally inhibited in the presence of 33 μm NS 1619. 10 NS 1619 (33 μm) inhibited the induction of I_K(ATP) by levcromakalim (10 μm). 11 It is concluded that NS 1619 activates the large conductance, Ca²⁺-sensitive channel, BK_Ca and over the same concentration range it inhibits both K_v and L-type Ca-channels. The observed NS 1619-induced mechanical inhibition in rat portal vein and aorta seems most likely to be due to the observed inhibition of Ca-currents.