Adrenal contribution to cardiac responses elicited by acute hypoxia in piglets

The adrenal contribution to cardiac responses elicited by acute hypoxia was assessed in 16 piglets, 1-12 wks old, anesthetized with pentobarbital (30 mg/kg). External cardiac work was held constant and parasympathetic blockade was produced in each animal with atropine (1 mg). Hypoxia was produced by addition of N2 to the respirator. In a sham-adrenalectomy group (n = 6) left ventricular (LV) dP/dtmax increased significantly during hypoxia (PaO2 approximately 30 mmHg) to 3,680 +/- 414 mmHg/s from control values of 2,686 +/- 317 mmHg/s (P < 0.01). Heart rate rose from 171 +/- 6 to 186 +/- 7 beats/min (P < 0.02). These responses were not significantly altered by ganglionic blockade with trimethaphan camsylate (0.5 mg x kg-1 x min-1). Equally large increases of LV dP/dtmax appeared when heart rate was held constant by pacing. beta-Adrenoreceptor blockade with practolol (4 mg/kg) sharply reduced but did not eliminate the response. In contrast, no changes in LV dP/dtmax or heart rate were observed during hypoxia in adrenalectomized piglets (n = 6). These findings indicate that the increased cardiac contractility during acute hypoxia in piglets is dependent on the integrity of the adrenal glands and that there is minimal contribution from cardiac sympathetic nerves.