Hypophysectomy alters cardiorespiratory variables: central effects of pituitary endorphins in shock

The possible involvement of pituitary endorphins in the pathophysiology of shock was evaluated by measuring cardiorespiratory variables after naloxone injection in conscious hypophysectomized and sham-hypophysectomized rats subjected to controlled hemorrhage. Additionally, the role of the central nervous system (CNS) in mediating the cardiodepressant effects of endorphins in shock was studied. After the induction of hypovolemic shock (20 min at below 40 mmHg), hypophysectomized and sham-hypophysectomized rats received intraventricular (ivt) injections of naloxone HCl (10 micrograms) or an equivalent volume of saline (20 microliters over 20 s). In sham-hypophysectomized rats, both injections significantly elevated mean arterial pressure and pulse pressure; however, the increase produced by naloxone was significantly greater than that produced by saline. By contrast, hypophysectomized rats showed no response to naloxone or saline. Intravenous (iv) administration of naloxone HCl (3 mg/kg) or saline to these same hypophysectomized rats 15 min after ivt administration had no additional cardiovascular effects; as before, only animals with intact pituitaries responded to naloxone. Heart rate and respiration rate were unaffected by ivt or iv naloxone. From these data we suggest that pituitary endorphins contribute to the pathophysiology of hypovolemic shock, at least in part through actions within the CNS.