Magnesium ions and oxytocin sensitivity of the male mouse anococcygeus

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.

Magnesium and biological activity of oxytocin analogues modified on aromatic ring of amino acid in position 2

For the purpose of evaluating substitution effects in the ortho, meta or para positions of the aromatic ring of tyrosine or phenylalanine in position 2 of oxytocin on uterotonic activity in vitro in the presence and absence of magnesium ions, six new analogues of oxytocin ([D- and L-m-methylphenylalanine2]oxytocin, [D- and L-m-methoxyphenylalanine2]oxytocin and [D- and L-o-methyltyrosine2]-oxytocin) were synthesized and several previously described analogues resynthesized. For the phenylalanine series, it is found that, in the absence of magnesium ions, substitution of the ortho and meta positions leads to loss of intrinsic activity (the analogues are antagonists) in contrast to the para position. In the tyrosine series, only methyl substitution in the meta position has this effect (substitution of ortho position only attenuates the agonistic biological activity). Addition of Mg ions restores to a certain degree the agonistic activity in the case of the o-methylphenylalanine analogue and enhances the agonistic activity of o-methyltyrosine oxytocin. All other analogues keep the original qualities as in the absence of Mg. Molecular modelling calculations of the structure of the above analogues was carried out to help explain these findings of the molecular level.

An oxytocin receptor in anococcygeus muscles isolated from male mice

The nature of the neurohypophyseal peptide receptor in the anococcygeus muscles from male mice was investigated. The rank order of potency of naturally occurring peptides was oxytocin greater than Arg-vasotocin greater than Arg-vasopressin greater than Lys-vasopressin, which is similar to that found in the uterus and mammary gland. Selective agonists on the oxytocin (OT) receptors of the uterus and mammary gland (Thr4-OT; Gly7-OT; Thr4-Gly7-OT) were also potent agonists in the mouse anococcygeus. Competitive antagonists of uterine responses to oxytocin (dP-TyrMe-Thr4-OT; dP-TyrMe-OT; dP-Thr4-OT; dp-Orn8-OT) were also competitive antagonists of oxytocin-induced contractions of the mouse anococcygeus. It is concluded that the neurohypophyseal peptide receptor of the male mouse anococcygeus is of the oxytocin type; antagonist pA2 values suggest that this receptor resembles, but may not be identical to, the uterine oxytocin receptor. Possible physiological and pharmacological implications of these observations are discussed.