An oxytocin receptor in anococcygeus muscles isolated from male mice

Biology of the anococcygeus muscle

The anococcygeus is a smooth muscle tissue of the urogenital tract which, in the male, runs on to form the retractor penis. The motor innervation is classically sympathetic with noradrenaline as transmitter, but the relaxant parasympathetic transmitter has only recently been identified as nitric oxide. Indeed, the anococcygeus has provided an extremely useful model with which to probe the mechanisms underlying this novel nitrergic system, including the importance of physiological antioxidants in maintaining the potency of nitric oxide as a neurotransmitter. The cellular mechanisms of contraction and relaxation are slowly being clarified, with particular interest in the contribution of capacitative calcium entry and the guanylyl cyclase/cyclic GMP system. Many questions remain unanswered, however, including the precise physiological role of the muscle, the identity of substances released from subcellular vesicles of nitrergic nerves, the unusual sensitivity of the tissue to certain peptides (oxytocin and urotensin II), and the nature of store-operated channels through which calcium enters the cell to maintain contraction.

Vascular vasopressin receptors

1. Vascular vasopressin receptors are understood because of the specific application of each major technical advance in pharmacology; this review shows that isolated organs, whole animal preparations, hormone synthesis, radioligand binding, and human studies have all played their part. 2. Even so, neither vascular vasopressin receptor heterogeneity nor occupancy-response relationships are fully understood; by way of comparison far more is known about alpha-adrenoceptors. 3. The biochemical pharmacology of vascular vasopressin receptor activation is still in its infancy. Whilst the second messenger molecules resulting from vascular vasopressin receptor activation appear to be component(s) of the pathways of phosphoinositide metabolism, technical difficulties have led investigators to study similar vasopressin receptors in other tissues. 4. It is not certain, for example, that results from hepatic vasopressin receptor studies can be automatically extrapolated to vascular smooth muscle. 5. Lastly, the directions that vascular vasopressin research might take are speculated on. It is not known whether the vascular vasopressin receptor is itself a polymer, whether receptor heterogeneity could be exploited in the clinical uses of vasopressins, nor whether vasopressins are co-released with other neurotransmitters.

Vasopressin receptor mediated contraction and [3H]inositol metabolism in rat tail artery

Inositol phosphates (IP) production and contraction in isolated but otherwise intact rat tail artery were measured in response to stimulation by vasopressin agonists. We have previously studied similar alpha-adrenoceptor responses. Identical rank orders of vasopressin agonists' potency were found for IP accumulation and contraction. The vasopressin analogue [1-(beta-mercapto-beta,beta-cyclopentamethylene propionic acid)-2-(O-methyl)tyrosine,D-arginine8] vasopressin, was shown to be a specific, reversible antagonist for both IP accumulation and contraction by all vasopressin agonists tested. No antagonism of vasopressin induced increases in IP accumulation or contraction were found using phenoxybenzamine. Therefore, in this tissue vasopressin receptors mediate both contraction and IP accumulation; and vasopressin mediated responses appear to be direct effects not mediated via the activation of alpha-adrenoceptors. Demonstration of two entirely different receptors, mediating the same functional response, and which both promote IP production, is consistent with a general obligatory role for phosphoinositide catabolism in receptor mediated vascular smooth muscle contraction.