Effects of naloxone on cardiovascular responses to static exercise and behavior in conscious cats

Conscious cats performing static (isometric) exercise were studied before and after administration of the opiate antagonist naloxone in order to evaluate the possible role of endogenous opiates in regulating cardiovascular responses to static exercise. Intravenous administration (0.4 mg/kg) of naloxone had no effect on resting left ventricular systolic pressure or on heart rate. In addition, similar increases in left ventricular systolic pressure and heart rate occurred during static exercise both before and after naloxone. However, naloxone reduced the number of exercise events the cats would perform per day. We conclude that moderate doses of naloxone do not modulate the cardiovascular responses to moderate static exercise in conscious cats. The results do indicate that endogenous opiates are involved in regulating the number of exercise bouts the cats will perform per day.