Freshly dissociated bladder smooth muscle cells of mice developed spontaneous, caffeine- (ICAF) and carbachol-induced (ICCh) currents under voltage-clamped conditions. Spontaneous currents, ICAF and ICCh were blocked with tetraethylammonium at 3 x 10(-4)-10(-2) M but were resistant to both charybdotoxin (10(-7)-10(-6) M) and iberiotoxin (10(-7)-10(-6) M). The reversal potential for each current indicated that K+ channels play a major role in the generation of each current. Both spontaneous currents and ICAF but not ICCh were abolished in nominally Ca2+-free and nicardipine (10(-6) M)-containing media. These results suggest that the activity of L-type voltage-sensitive Ca2+ channels is important in the generation and maintenance of spontaneous currents and ICAF but not ICCh. Ryanodine (10(-6) M) prevented spontaneous currents, ICAF and caffeine-induced [Ca2+]i elevation but not ICCh and carbachol-induced [Ca2+]i elevation, suggesting that the response of bladder smooth muscle cells to carbachol may involve a Ca2+ store distinct from that for caffeine. Pretreatment with carbachol suppressed ICAF to 22 +/- 7% (n = 7) and the caffeine-induced [Ca2+]i elevation to 25 + 3% (n = 6). Similarly, caffeine suppressed ICCh to 23 +/- 4% (n = 9) and the carbachol-induced [Ca2+]i elevation to 24 +/- 6% (n = 6).