Biphasic response of the rabbit detrusor muscle to changing the extracellular concentrations of potassium and calcium

In the presence of 1.8 mM calcium (normal Tyrode's solution) increasing the potassium concentration significantly enhanced the phasic response of the rabbit detrusor muscle to 1 Hz field stimulation but did not affect either the tonic tension or the maximal rate of contraction. 2. At low calcium (0.6 mM) increasing the potassium concentration enhanced the phasic response to 1 and 4 Hz and significantly increased the tonic tension and the rate of tension generation. 3. Increasing the extracellular calcium concentration enhanced all parameters in a dose-dependent manner at all frequencies. The effect of increasing calcium was significantly greater at 1 and 4 Hz than at 32 Hz. Surface spectrofluorometry demonstrated that the increase in intracellular calcium stimulated by field stimulation paralleled the phasic contractile responses. 4. These results can be explained by potassium-induced increases in the intracellular bound calcium.