RESPONSIVENESS TO SYNTHETIC PARATHYROID-HORMONE IN THE PORTAL-VEIN OF PORTAL HYPERTENSIVE RATS

In addition to its hypotensive action, the parathyroid hormone also decreases portal pressure in portal hypertensive rats. The purpose of this study was to characterize the vascular effects of the parathyroid hormone on the portal vein of portal vein-stenosed rats. When intravenously infused at the rate of 1.62 x 10(-11) mol/kg per min, the parathyroid hormone lowered portal pressure (15.1 +/- 0.7 vs. 14.0 +/- 0.8 mmHg) without affecting systemic blood pressure. With the portal vein isolated, the parathyroid hormone shifted the dose-response curves of KCl and acetylcholine to the right. However, the vasodilator effect of the parathyroid hormone was significantly less in portal hypertensive rats (EC50 Of KCl increased 129.2% and acetylcholine 199.3%), compared to sham-operated rats (EC50 of KCl increased 158.7% and acetylcholine 270.2%). Similar results were found for the vasodilator action of verapamil (10(-9)-10(-6) M). On the other hand, the vasodilator effect of forskolin was similar for both groups. These results suggest that decreased responsiveness to the parathyroid hormone may be associated with calcium utilization by vascular smooth muscles.