The effects of various anaesthetics on rat vasopressin release (author's transl)

Abstract

Most of the reported studies on the effects of various anaesthetics on vasopressin release have been done based on observations of indirect parameters or bioassay of the hormone, and the results have been quite controversial. The present studies were under-taken, therefore, to evaluate the effect of various anaesthetics on vasopressin release in vivo and in vitro with a radioimmunoassay which we have recently developed.The antibody against arginine vasopressin was raised by immunizing rabbits with lysine vasopressin conjugated with bovine thyroglobulin. The antisera had more than a 90% cross reactivity to arginine vasopressin and DDAVP but virtually no cross reactivity to oxytocin, ACTH, TSH and TRH. Arginine vasopressin was radioiodinated by the chloramine-T method. The radioimmunoassay procedure was as follows : 0.1ml of standard vasopressin or sample, 0.05ml of antiserum (1 : 5,000), 0.1ml of 0.01M phosphate buffer (pH 7.4) and 0.05ml of 125I-AVP (2,500cpm) were incubated for 72 hours at 4°C, and polyethylene glycol was used for the separation of bound and free hormone. The least detectable value in this sytem was 1.5 pg/ml.Ethyl carbamate (1 or 2 g/kg B.W.) or pentobarbital (35 or 60mg/kg B.W.) was injected intraperitoneally into Wistar rats. Ether was administered using an anaesthetic bottle. Blood specimens were obtained from animals by decapitation 20 minutes after the administration of anaesthetics, and vasopressin was extracted from plasma with cold acetone and petroleum-ether successively. After removal of acetone and petroleum-ether under a stream of air, the samples were reconstituted with an assay buffer and served for RIA.The plasma vasopressin level in the anaesthetized rats (except for those which received 1 g/kg B.W. of ethyl carbamate) was 3-7 times higher than that in the control rats which received intraperitoneal saline. The plasma osmolality was also elevated in all experimental groups except those treated with ether and ethyl carbamate (1g/kg B.W.), although there was no correlation between plasma vasopressin level and osmolality.When the neural lobes were examined in vitro following incubation in a Krebs-Ringer bicarbonate buffer containing ethyl carbamate (1 or 2 mg/ml) or pentobarbital (0.05 or 0.1mg/ml), there was no significant change in vasopressin release as compared to control tissues incubated in a plain Krebs-Ringer bicarbonate buffer, although viability of the tissues was proved by a marked increase in hormone release during the final exposure to 60mM KCl.In summary, ether, ethyl carbamate and pentobarbital induced the increase of plasma vasopressin levels in rats in vivo but had no effect on vasopressin release in vitro. These results indicate that these anaesthetics have no direct stimulous effect on the neural lobe, and the elevation of plasma vasopressin levels during anaesthesia is caused through unknown mechanisms such as a change in plasma volume or osmolality, alternation of sensitivity of receptors to those stimuli and possibly a direct stimulation of hypothalamic nuclei. For studies of vasopressin release in rats, ethyl carbamate (1g/kg B.W.) was found to be the most suitable anaesthetic among those presently tested.