Hemodynamic and Endocrine Parameters in the Anesthetized Cynomolgus Monkey: A Primate Model

Hormonal responses to surgical stress

Surgical stress is associated with activation of both the HPA axis and the sympathetic nervous system. Emergence from anesthesia appears to be a far more potent stimulus for both of these axes than the surgery itself. Intraoperative HPA activation can be moderate in degree and the mild cortisol elevation observed is compatible with an uneventful perioperative course. This observation was confirmed in adrenalectomized nonhuman primates where a physiologic replacement dose of cortisol was both necessary and sufficient to tolerate surgical stress. Taken together these human and nonhuman primate data suggest that the role of glucocorticoids during surgery is primarily permissive. Catecholamines appear to act in synergy with glucocorticoids during surgical stress.

Catecholamine-glucocorticoid interactions during surgical stress

The stress response involves activation of the hypothalamic-pituitary adrenal axis and the sympathetic nervous system. To study the relative contributions of glucocorticoids, epinephrine, norepinephrine, and dopamine to homeostasis, we examined the effects of cortisol and epinephrine deficiency on the norepinephrine and dopamine responses to surgical stress in nonhuman primates. Adult male cynomolgus monkeys (n = 7-8/group) underwent bilateral or sham adrenalectomy and were maintained for 4 months on physiologic glucocorticoid (hydrocortisone phosphate, 32 mg/M2/day) and mineralocorticoid (DOCA pivalate, 1 mg/day) replacement, or placebo injections, respectively. The adrenalectomized monkeys were then stratified into three groups receiving subphysiologic (X1/10), physiologic (X1), or supraphysiologic (X10) glucocorticoid replacement. A cholecystectomy was performed 4 days later and the pre-, intra-, and postoperative plasma catecholamine levels were measured. Adrenalectomized animals had markedly decreased plasma epinephrine concentrations and accentuated plasma norepinephrine and dopamine responses to cholecystectomy, when compared to sham-adrenalectomized controls. The magnitude of the norepinephrine accentuation was inversely related to the dose of cortisol administered preceding surgery. Norepinephrine and dopamine failed to completely normalize even at the high dose of glucocorticoid replacement which had been calculated to be similar to the cortisol production rate during maximal stress. Thus, surgical stress in the setting of inadequate glucocorticoid (cortisol) replacement and epinephrine deficiency, stimulates additional sympathetic activity, probably as an adaptive mechanism to maintain homeostasis. Both norepinephrine and dopamine are recruited to compensate for the glucocorticoid and epinephrine deficiency. Glucocorticoid administration decreases but does not normalize this compensatory sympathetic activity.

Hemodynamic effects of corticotropin releasing hormone in the anesthetized cynomolgus monkey

Right atrial bolus administration of rat/human corticotropin releasing hormone (r/hCRH) at a dose of 90 micrograms/kg to anesthetized cynomolgus monkeys caused a dramatic and prolonged fall in both the peripheral vascular resistance (48% reduction) and mean systemic blood pressure (36% reduction). An associated tachycardia could be blocked with prior propranolol administration and thus was probably reflexic. A mean 43 and 37% increase in the flow of the superior mesenteric and common iliac arteries, respectively, was demonstrated with electromagnetic flow probes. These changes were associated with a concomitant 38 and 40% diminution in the respective vascular resistance. Similar blood flow changes were noted in the carotid artery, however, these were of a much shorter duration. None of these changes occurred in placebo-treated animals. Plasma adrenocorticotropic hormone and cortisol concentrations were elevated basally and throughout the procedure and were similar in the experimental and control groups, suggesting maximal activity of the hypothalamic-pituitary-adrenal axis. Plasma renin activity, however, gradually increased in the r/hCRH-treated animals, probably as a result of the systemic hypotension. We speculate that CRH or a CRH-like substance may function as a paracrine hormone modulating local blood vessel tone and may be important in directing blood flow during stress and injury. The vasoactive properties of exogenous r/hCRH may be of clinical use in man.