Vagal and sympathetic components of the heart rate reflex in chronic portal vein stenosis.

The effects of portal hypertension and portosystemic shunting on autonomic components of the heart rate (HR) baroreflex and on skeletal muscle blood flow changes were investigated using the chronic portal vein-stenosed rat. Phenylephrine- and sodium nitroprusside-induced changes in mean arterial pressure (MAP), HR, and skeletal muscle conductance (SMC) were assessed before and after muscarinic or beta-adrenoceptor blockade. Stenosed rats had lower MAP than sham-operated rats (90 +/- 3 vs. 81 +/- 2 mmHg, P < 0.05), and their portal pressure was higher (7.4 +/- 0.5 vs. 13.9 +/- 1.0 mmHg, P < 0.005). Phenylephrine pressor responses were reduced in stenosed animals, their associated bradycardic responses were enhanced [-1.912 +/- 0.109 vs. -1.427 +/- 0.148 beats per minute (bpm)/mmHg, P < 0.01], and their SMC responses were diminished. Methylatropine abolished bradycardic responses and enhanced pressor responses without affecting SMC. After propranolol, reflex bradycardic responses in stenosed rats were less than in shams (-0.492 +/- 0.085 vs. -0.738 +/- 0.058 bpm/mmHg, P < 0.01), and their pressor and SMC responses became indistinguishable from shams. In contrast, tachycardic responses to nitroprusside-induced hypotension before propranolol were impaired in stenosed rats (-1.492 +/- 0.114 vs. -2.225 +/- 0.347 bpm/mmHg, P < 0.05), and their SMC responses were reduced. Muscarinic blockade did not affect HR or SMC responses to hypotension in either stenosed or sham rats. beta-Adrenoceptor blockade, however, prevented hypotension-induced tachycardia, enhanced nitroprusside depressor responses, and eliminated the between-group differences in SMC responses. These studies indicate that increased gain of the parasympathetic limb of the cardiac baroreflex was responsible for attenuated pressor responses to phenylephrine in portal vein-stenosed animals and that beta-adrenoceptors contributed to skeletal muscle vascular hyporesponsiveness to phenylephrine in portal-vein stenosed animals. Altered beta-adrenoceptor function also appears to contribute to impaired chronotropic and skeletal muscle conductance responses to hypotension.