STIMULATION OF ENDOGENOUS ENDOTHELIN RELEASE IN THE ANESTHETIZED RAT

Although many agents have been shown to stimulate release of endothelin (ET) in vitro, the factors regulating and affecting production and release of ET in intact animals remain obscure. Experiments were conducted to determine the effect of surgical preparation and infusion of plasma and/or saline on circulating immunoreactive (ir)-ET concentrations and to characterize the mechanism of endotoxin-induced increases in plasma ir-ET. Male Sprague-Dawley rats (210-325 g) anesthetized with Inactin were placed on a surgical heating table for a period of about 2 h before blood was collected via puncture of the abdominal aorta. The plasma concentration of ir-ET was 8.1 +/- 0.9 pg/ml in rats that did not undergo surgical preparation or receive any further treatment. Standard surgical preparation, as for renal clearance experiments (catheters in the trachea, femoral artery, femoral vein, and bladder), resulted in a dramatic rise in ir-ET to 17.7 +/- 3.0 pg/ml (p < 0.05). Infusion of plasma from donor rats (5 ml/kg over 20 min) resulted in an additional significant increase to 32.6 +/- 3.5 pg/ml (p < 0.05). In contrast, infusion of saline (0.9% NaCl) in a similar manner produced no further increase in ir-ET levels (23.2 +/- 3.2 pg/ml). Infusion of endotoxin in anesthetized, surgically prepared rats significantly increased ir-ET. This increase was not blocked by a platelet-activating factor antagonist (Esai 6123), despite blockade of endotoxin-induced hypotension. Phosphoramidon, which has been shown to block in vivo conversion of exogenous big ET to ET, was unable to prevent endotoxin-induced increases in ir-ET. Our findings demonstrate that standard small-animal experimental techniques, such as surgery and plasma infusion, produce significant changes in circulating ET. Furthermore, these data support the idea that endotoxin-induced increases in irET are not mediated by platelet-activating factor or a phosphoramidon-sensitive endothelin converting enzyme.