Neuroeffector transmission to different layers of smooth muscle in the rat penile bulb

1. Using intracellular recording techniques, two distinct layers of smooth muscle were identified in the rat penile bulb. The inner muscle layer (parenchyma) exhibited spontaneous action potentials, while the outer sheet (sac) was electrically quiescent. 2. In the parenchyma, transmural stimulation initiated non-adrenergic, non-cholinergic (NANC) inhibitory junction potentials (IJPs) which were abolished by Nomeganitro-L-arginine (LNA) or 1H-[1,2, 4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). The amplitude of IJPs was reduced by ouabain, dinitrophenol or decreasing the extracellular potassium concentration ([K+]o) but not by several K+ channel blockers. 3. The parenchyma also received an excitatory innervation mediated by alpha-adrenoceptors which caused a contraction that was not associated with a membrane potential change. 4. In the sac, transmural stimulation initiated two component excitatory junction potentials (EJPs) mediated by alpha-adrenoceptors and associated action potentials. The initial component was more dramatically suppressed than the secondary component by caffeine, ryanodine or cyclopiazonic acid (CPA). Lowering of the extracellular chloride concentration ([Cl-]o) selectively inhibited the rapid component of EJPs, while niflumic acid was less potent. 5. These results suggest that IJPs in the parenchyma result from the release of NO which stimulates sodium pump activity following the activation of guanylate cyclase. In the sac, the activation of alpha-adrenoceptors initiates EJPs by releasing Ca2+ from intracellular stores which activates Ca2+-activated channels.