ELECTRICAL-PROPERTIES OF SMOOTH-MUSCLE IN THE GUINEA-PIG URINARY-BLADDER

1. The effects of transmural nerve stimulation were examined on prepar ations of detrusor smooth muscle from guinea-pig urinary bladder using intracellular recording techniques. Most recordings were made from pr eparations in which spontaneous and evoked action potentials had been inhibited by nifedipine (10 mu M), a dihydropyridine that blocks L-typ e Ca2+ channels. 2. Supramaximal stimuli evoked excitatory junction po tentials (EJPs) which could be divided into three basic types. Type 1 EJPs had short latencies (<30 ms) and fast rise times (<60 ms). Type 2 EJPs consisted of two components: a small depolarization that was fol lowed by a second depolarization with a faster rise time. In a third t ype of cell, at high strengths of stimulation, EJPs resembled type 1 E JPs but at lower strengths of stimulation were similar in time course to type 2 EJPs. 3. All EJPs were abolished by tetrodotoxin (1 mu M) an d reduced by omega-conotoxin (0.1 mu M), but were unaffected by hexame thonium (0.1 mM), suggesting that they result from the release of tran smitter from post-ganglionic nerve fibres. All responses persisted in the presence of atropine (1 mu M) but were abolished following the des ensitization of P-2-purinoceptors with alpha,beta-methylene ATP (m-ATP ; 10 mu M). 4. Spontaneous excitatory junction potentials (SEJPs) were also recorded fr om most cells SEJPs were similar in appearance to fa st single-component EJPs; however, in general they had a briefer time course. SEJPs persisted in the presence of tetrodotoxin (1 mu M). 5. T he electrical proper ties of urinary bladder smooth muscle were also e xamined. Voltage changes induced by point current injection into cells had fast rates of rise and decay (time constant, 5-20 ms). The input resistance of cells ranged between 12 and 108 M Omega. When recordings were taken from cells near the point of current injection, resultant electrotonic potentials could be detected in only a small proportion o f cells. 6. The results are discussed in relation to the idea that tra nsmural nerve stimulation in the guinea-pig urinary bladder causes the activation of at least two different membrane conductances. Cells app ear to be electrically coupled with one another. However, it is likely that coupling exists within discrete bundles of the smooth muscle.