Synthetic analogues of oxytocin acting as hormonogens

Modes of inactivation of neurohypophysial hormones: significance of plasma disappearance rate for their physiological responses

Analogues of oxytocin and deaminooxytocin with 4-glutamine replaced by 4-glutamic acid methyl ester readily lose their uterotonic activity when incubated with rat serum, presumably by hydrolysis to the much less active 4-glutamic acid derivatives. On the other hand, inactivation of the deaminooxytocin analogue in the rat uterus, as demonstrated by the "oil-bath" technique, is only slightly more rapid than that of deaminooxytocin and distinctly slower than that of oxytocin. Its in situ/in vitro ratio of uterotonic activity is less than 0.1 whereas that for deaminooxytocin is about 3 and also the peristence of the uterotonic effect in situ is slightly less than that of deaminooxytocin. The results with these "rapidly inactivated" analogues can be used as proof of some predictions of the three-compartment model for tissue distribution of neurohypophysial hormones and its influence upon the time course of a biological response published earlier. The potential use of analogues of neurohypophysial hormones as probes for inactivation mechanisms and the results thus far obtained are discussed.

The action of scretin, cholecystokinin-pancreozymin and caerulein on pancreatic secretion in the rat

1. The actions of secretin, cholecystokinin-pancreozymin (CCK-PZ) and caerulein have been studied on the rate of flow and protein secretion from the pancreas of urethane-anaesthetized rats.2. Secretin stimulated the rate of flow of pancreatic juice and also stimulated the secretion of small but significant quantities of protein. Cholecystokinin-pancreozymin and caerulein strongly stimulated both the rate of flow and rate of protein secretion.3. Maximal peak rates of flow of juice stimulated by CCK-PZ (20.3 mul./5 min), were significantly higher than stimulated by secretin (14.0 mul./5 min), but maximal volumes of pancreatic juice secreted in total response to rapid I.V. injections of secretin and CCK-PZ were not significantly different.4. The time course of rates of flow of juice stimulated by CCK-PZ and caerulein were different to those stimulated by secretin. With CCK-PZ and caerulein peak rates occurred earlier and responses declined sooner than with secretin.5. The actions of CCK-PZ, caerulein and secretin on the pancreas of the rat are discussed in the light of the responses evoked in other species. It is concluded that the rat is less sensitive than other spieces to secretin but not CCK-PZ.

Some pharmacological properties of a synthetic oxytocin analogue [1-N-carbamoyl-hemicystine-2-O-methyltyrosine]-oxytocin (carbamoyl-methyloxytocin), an antagonist to the neurohypophysial hormones

1. A synthetic oxytocin analogue, [1-N-carbamoyl-hemicystine-2-O-methyltyrosine]-oxytocin (carbamoyl-methyloxytocin), has been tested as an antagonist to the actions of oxytocin and vasopressin on the uterus, the mammary gland and blood pressure.2. The analogue inhibited the response of the isolated rat uterus to both oxytocin and vasopressin without itself stimulating the uterus to contract. The responses to equipotent doses of oxytocin and vasopressin were inhibited equally. There was little or no inhibition of the response to bradykinin. carbachol, angiotensin or 5-hydroxytryptamine with doses of the analogue up to 160 times that required to inhibit the response to oxytocin by 50%. The analogue caused a parallel displacement of the log dose-response curve for oxytocin; the pA(2) value (2 min contact) varied from 6.4 to 7.1 according to the ionic composition of the solution in the organ bath.3. The analogue inhibited the response of the rat uterus in situ to oxytocin but not to angiotensin or 5-hydroxytryptamine. It did not stimulate the uterus.4. When, in certain experimental conditions, spontaneous activity occurred in the isolated uterus or the uterus in situ, this activity was unaffected by the analogue but the increase in amplitude and frequency of contractions caused by oxytocin was inhibited. The regular rhythm of contractions induced in the quiescent uterus by the intravenous infusion of oxytocin was interrupted by intravenous injections of the analogue.5. The response of the isolated strip of rat mammary gland to the analogue depended on whether or not magnesium was present in the bath solution. In the presence of this ion, the analogue generally caused an increase in tension; in its absence, it acted as a pure antagonist. As on the isolated uterus, oxytocin and vasopressin were equally inhibited, and the analogue caused a parallel displacement of the log dose-response curve for oxytocin. With 0.9 mM Ca and 1.0 mM Mg, the mean pA(2) value (2 min contact) was 6.28 +/- 0.08 (S.E.)6. In the lactating rat, the analogue inhibited the milk-ejection response to oxytocin and vasopressin but not that to acetylcholine, bradykinin or 5-hydroxytryptamine. A milk-ejection response to the analogue itself was seen occasionally with retrograde arterial but not with intravenous injections.7. The analogue inhibited the avian depressor response to oxytocin and the rat pressor response to vasopressin.8. On all assay preparations, the degree of inhibition caused by the analogue was dependent on the dose, and the inhibition could be surmounted by increasing the dose of agonist. Recovery usually occurred within 15 min. These features, together with the parallel displacement of the dose-response curve for oxytocin on the isolated uterus and mammary strip, and the equal inhibition of the responses to oxytocin and vasopressin, suggest that carbamoyl-methyl-oxytocin acts as a specific competitive inhibitor of the neurohypophysial hormones.9. The structure-activity relationships of analogues of oxytocin having substituents in the terminal amino and phenolic hydroxyl groups, and some practical applications of the carbamoyl-methyl analogue, are discussed.

Factors influencing the sensitivity of the rat to vasopressin

1. Indirect evidence suggests that the concentration of arginine vasopressin in the plasma of normally hydrated man is about 1 mu-u./ml., but this is usually considered to be below the limit of sensitivity of the standard assay preparation, the water-loaded Wistar rat under ethanol anaesthesia.2. It was found that there was a surprising variation in sensitivity to vasopressin between batches of Wistar rats, and that other varieties of rat (including those with diabetes insipidus) were no more sensitive.3. Three modifications of the standard assay procedure produced an increase in sensitivity:(a) using Wistar rats weighing 100-150 g, rather than larger animals;(b) commencing the assay shortly after surgery, i.e. as soon as the urine flow reached 25 mul./min;(c) infusing vasopressin intravenously (0.5-3 mu-u./min). By using modification (a) with either (b) or (c) it was possible to detect as little as 0.5 mu-u.4. With these modifications antidiuretic activity equivalent to 0.5-2.0 mu-u./ml. of arginine vasopressin was measured in nine samples of plasma from a normally hydrated subject.5. It is suggested that the frequent reports of enhanced sensitivity may have been due to the fortuitous use of a particularly sensitive batch of rats, or to a high endogenous secretion of vasopressin due to operative trauma.

The assay of milk-ejecting activity in the lactating rat

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