DESENSITIZATION OF THE CARDIAC RESPONSE TO SOMATOSTATIN IN ISOLATED TOAD ATRIA BUT NOT IN WHOLE TOADS

The cardiac slowing measured in response to repeated application of so matostatin (SOM) was compared in an isolated toad sinus venosus/ atria l preparation and in a whole, anaesthetised toad, Bufo marinus. Repeat ed doses of 0.5-1 X 10(-7) M SOM in the organ bath progressively reduc ed the cardiac response to 25% or less of the response to the first do se given. When preparations were allowed a recovery period of 50-80 mi n, the response recovered to 51.0 +/- 3.7% of the control. In contrast , the cardiac chrontropic response in anaesthetised toads to repeated intravenous injections of 2 mug or 1.2 nmol SOM over 3 h was not chang ed. Prolonged exposure to SOM (0.1-0.25 mumol/kg/h infusion), for up t o 4 h in anaesthetised toads caused no significant reduction either in the cardiac responses to a bolus dose of SOM (0.6 nmol) or nerve rele ased SOM (3 Hz for 2 min) in the presence of atropine. These opposing results suggest that the half-life of a peptide and the method of expo sing receptors to a peptide are important factors in receptor desensit isation.