Pudendal nerve stimulation of a preparation of isolated guinea-pig urethra

To assess the functional response of the urethral striated muscle to activation of its nerves, using a novel isolated organ-bath preparation.

The urethra of the female guinea-pig was chosen as a suitable model for investigation, as it is functionally and structurally similar to the human urethra. Female Dunkin-Hartley guinea-pigs (400-500 g) were used; for the histochemical and immunohistochemical experiments, unfixed urethras were cryo-sectioned (14 microm thick) and were stained using established methods. For in vitro experiments, whole urethras were suspended vertically, with pudendal nerves intact, for isometric tension and intraluminal pressure recording in a 40-mL organ bath. Drugs were applied directly to the bathing solution.

In the striated muscle layer of the urethra there was positive beta-NADPH-diaphorase activity. In organ-bath studies the pudendal nerve-evoked contractions (0.2 ms pulses, 5 s trains, 70 V and 1-100 Hz) were abolished in the presence of tubocurarine (10(-6)m), and unaffected by guanethidine and atropine (both 10(-6)m). Pre-incubation with sodium nitroprusside and SIN-1 chloride significantly reduced the initial peak pressure responses (P < 0.05, anova for paired data) evoked by electrical field stimulation of the pudendal nerves at stimulus parameters of 0.2 ms pulses, 5 s trains, 70 V and 25 Hz.

Electrically induced contractions were abolished by tubocurarine, confirming that the pudendal nerve innervates the striated muscle of the guinea-pig external urethral sphincter via nicotinic receptors. beta-NADPH-diaphorase histochemistry gave positive staining around guinea-pig striated muscle cells and possibly identified neuromuscular junction sites staining positively for the nitric oxide synthase marker. Together with the results of the organ-bath experiments, the results suggest that the striated muscle cells of the guinea-pig urethra have the machinery to respond to nitric oxide.