Maximal flux responses after multiple challenges with vasopressin.

Antidiuretic hormone (ADH) increases transepithelial flux of water and particular solutes across the amphibian urinary bladder and mammalian collecting duct by increasing the permeability of the apical surface. We find that if each challenge with ADH is ended by replacing the medium bathing both the mucosal and serosal surfaces of the toad bladder, then rechallenge with the same supramaximal dose of ADH 36-100 min later produces flux equivalent to or greater than the original response, but rechallenge after 15 min produces only 68% of the original response. If the medium bathing the mucosal surface is neither replaced nor returned to its original volume, complete recovery of the osmotic flux response to ADH does not occur. Maximal restimulation by ADH occurs with transepithelial osmotic gradients between 119 and 180 mosmol/kg during both challenges (the serosal bath is always isotonic amphibian Ringers). In addition, ADH-containing serosal baths that have maximally activated transport across bladders for 30-60 min can be reused and again produce maximal activation of ADH responses in fresh bladders or in the original bladders after washing. These results are in contradistinction to reports of desensitization of transepithelial flux upon rechallenge with ADH after an initial stimulation under many conditions. Our findings suggest that desensitization in vitro may result from experimental design rather than intrinsic biological characteristics of the system.

Reproductive performance of Polwarth and Corriedale sheep in Tasmania.

The reproductive performance of 27 Polwarth and 9 Corriedale flocks was studied under commercial conditions in Tasmania. Mean ovulation rate was 149% but the mean number of lambs marked per ewe joined was only 88%. The major sources of wastage contributing to the overall loss were identified as an 18% loss of embryos in twin-ovulating ewes which gave birth to only one lamb, and a mean perinatal lamb mortality of 21% of lambs born. There was no significant difference in embryo survival between unilateral and bilateral twin-ovulating ewes.