Histological and functional aspects of different regions of the rabbit vagina.

Relatively little data is available on regional differences in both morphological and functional studies of vaginal smooth muscle in the rabbit. Histological and in vitro strip studies were performed upon the vaginal walls of 10-week-old New Zealand White rabbits discriminately according to location (upper, middle or lower) and the type of muscle arrangement (longitudinal or circular). The contraction and relaxation responses of the vaginal smooth muscle were assessed. The upper and middle third of the vagina were histologically characterized by an abundance of smooth muscle, while the lower third was composed of numerous sinusoids scattered among smooth muscle bundles. The peak amplitudes of 60 mM KCl-induced contraction per tissue strip weight were regionally different (upper>middle>lower). Contractile responses induced by adrenergic agonists (epinephrine, norepinephrine, phenylephrine and isopreterenol) showed regional differences, and were mainly of a tonic nature in lower part and strong phasic in middle and upper vagina. Electrical field stimulation induced a prominent relaxation response in the lower third of the vagina precontracted with phenylephrine (5 microM). This relaxation response was partially inhibited by N-omega-nitro-L-arginine methyl ester (NAME, 43.3+/-6.9%, s.e.), a nitric oxide synthase inhibitor, and D-vasoactive intestinal peptide (VIP) (11.3+/-4.4%), a VIP receptor antagonist. The type of muscle arrangement did not affect the results. Our results demonstrated that rabbit vagina has regional difference, not only histologically but also functionally. The contractile response was induced in all regions of the rabbit vagina by sympathetic agonists, while the prominent nerve-mediated relaxation was identified in the lower third portion of the rabbit vagina. The nature of relaxation and contractile function of vagina awaits future investigation.