Inhibitory effect of high [Mg2+]on the vasopressin-stimulated hydroosmotic permeability of the isolated perfused cortical collecting duct

High magnesium concentration inhibits the effect of arginine vasopressin (AVP) on smooth muscle contraction and platelet aggregation and also influences hepatocyte AVP receptor binding. The aim of this study was to determine the role of magnesium concentration [Mg2+] in AVP-stimulated water transport in the kidney collecting duct. The effect of low and high peritubular [Mg2+&] on the AVP-stimulated osmotic water permeability coefficient (Pf) was evaluated in the isolated perfused rabbit cortical collecting duct (CCD). Control tubules bathed and perfused with standard Ringer bicarbonate solution containing 1 mM Mg2+ presented a Pf of 223.9 + OR - 27.2 µm/s. When Mg2+ was not added to the bathing solution, an increase in the AVP-stimulated Pf to 363.1 + OR - 57.2 µm/s (P<0.05) was observed. An elevation of Mg2+ to 5 mM resulted in a decrease in Pf to 202.9 + OR - 12.6 µm/s (P<0.05). This decrease in the AVP-stimulated Pf at 5 mM Mg2+ persisted when the CCDs were returned to 1 mM Mg2+, Pf = 130.2 + or - 20.3 µm/s, and was not normalized by the addition of 8-[4-chlorophenylthio]-adenosine 3',5'-cyclic monophosphate, a cAMP analogue, to the preparation. These data indicate that magnesium may play a modulatory role in the action of AVP on CCD osmotic water permeability, as observed in other tissues

Effect of propranolol and nifedipine on in vitro fluid absorption by the isolated perfused proximal convoluted tubule of the rabbit

The inhibition of fluid absorption (Jv) by the antiarrhythmic and antihypertensive drugs propranolol and nifedipine, which increase cytosolic Ca*+ concentration, was studied using the isolated rabbid proximal convoluted tubule perfused in vitro. Proximal convuluted tubules were perfused and bathed with a modified Krebs-Henseleit solution containing bovine serum albumin. Jv was measured after a 30-min control period, after 40 min with eithe 0.1 mM propranolol or 1.0 mM mifedipine on the peritubular side and after a 40-min recovery period. Both drugs inhibited Jv (58% propranolol, and 21% nifedipine) The 40-min recovery period was sufficient to reserve the effect of nifedipine, but propranolol-treated tubules (N=6) only reached 78% of the control Jv value. These results demonstrate that antiarrhythmic and anthypertensive drugs are powerful inhbitors of net fluid absorption by exerting a direct effect on proximal or distal tubule cells, thus acting like "local diuretics"