Calcium dependence of BAY K 8644 effects on the rabbit gall-bladder

In the present study, we characterized the effects of the calcium (Ca2+) channel activator BAY K 8644 on sodium (Na+) absorption and transepithelial potential difference (Pd) in the rabbit gall-bladder. In gall-bladders mounted in an Ussing chamber it was observed that serosal BAY K 8644 (10(-5) M) inhibited Na+ absorption in the presence, but not in the absence of serosal Ca2+. Serosal nifedipine (a Ca2+ channel antagonist) at 10(-5) M did not reverse the Na+ transport inhibition caused by BAY K 8644. Another effect of serosal BAY K 8644 (10(-5) M) was to induce oscillations in Pd. These Pd-oscillations had a frequency of about one per minute and an amplitude of 20-40 microV. The appearance of Pd-oscillations was dependent on the presence of Ca2+ in the serosal medium. The oscillations were abolished by 1-3 x 10(-5) M serosal nifedipine and by bilateral application of 3 mM barium (Ba2+) (a K+ channel blocker). In a sac preparation of the rabbit gall-bladder, spontaneous cyclic contractions of smooth muscle cells in the gall-bladder wall were observed as oscillations in the transmural pressure. These spontaneous contractions were not accompanied by oscillations in Pd. Serosal BAY K 8644 (10(-5) M) evoked oscillations in Pd in half of the sac preparations, but in each gall-bladder the frequencies of Pd-oscillations and pressure oscillations were different. Serosal nifedipine (2 x 10(-5) M) abolished both types of oscillation.(ABSTRACT TRUNCATED AT 250 WORDS)