Electrical and mechanical responses of guinea-pig bladder muscle to nerve stimulation

The electrical and mechanical responses to transmural stimulation of intrinsic nerves have been recorded from smooth muscle strips dissected from the dome of the guinea-pig bladder, by use of intracellular microelectrodes, and conventional tension recording techniques. Stimulation of intrinsic nerves evoked action potentials in all cells studied. Hyperpolarization of the cells by extracellular current injection revealed subthreshold excitatory junction potentials (e.j.ps) in about a quarter of the cells studied. Action potentials could still be evoked in the presence of atropine and neostigmine, but were abolished after desensitization of the cells to α,β-methylene ATP, a stable analogue of ATP. In the presence of neostigmine, the evoked action potential was followed by a slow depolarization of the membrane. The mechanical response increased in amplitude and duration. The contractile response to transmural nerve stimulation was reduced but not abolished in the presence of either atropine or desensitizing doses of α,β-methylene ATP. Atropine was more effective at high frequencies of stimulation (≥ 30 Hz), and α,β-methylene ATP at low frequencies (≤ 15 Hz). In combination the drugs abolished the response. The results suggest that the mechanical response to excitatory nerve stimulation is biphasic. The early transient response is elicited by e.j.ps and evoked spikes, is resistant to atropine, but sensitive to desensitization of purinoceptors. The late response is mediated through muscarinic receptors, involves little membrane depolarization, and is unaffected by desensitization of purinoceptors. These responses are analogous to the responses seen in rabbit bladder, and in the sympathetically innervated rat tail artery and guinea-pig vas deferens.