Mechanism of norepinephrine release elicited by renal nerve stimulation, veratridine and potassium chloride in the isolated rat kidney.

We have investigated the mechanism by which renal nerve stimulation (RNS), veratridine (Vt) and KCl promote release of norepinephrine in the isolated rat kidney perfused with Tyrode's solution and prelabeled with [3H]norepinephrine by examining the overflow of tritium elicited by these stimuli during 1) extracellular Ca++ depletion, 2) alterations in extracellular Na+ concentration and 3) administration of tetrodotoxin, amiloride, LiCl and calcium channel blockers. RNS (1-4 Hz), Vt (15-90 nmol) and KCl (150-500 mumol) produced renal vasoconstriction and enhanced the tritium overflow in a frequency- and concentration-dependent manner, respectively. Omission of Ca++ (1.8 mM) from the perfusion fluid abolished the renal vasoconstriction and the increase in tritium overflow elicited by RNA and KCl and substantially reduced that caused by Vt. Lowering the Na+ concentration in the perfusion medium (from 150 to 25 mM) reduced the overflow of tritium and the renal vasoconstriction caused by RNS (2 Hz) or Vt (45 nmol); the increase in tritium overflow in response to these stimuli was positively correlated with extracellular Na+ (25-150 mM). In contrast, KCl-induced tritium overflow was negatively correlated with extracellular Na+ concentration. Tetrodotoxin (0.3 microM) abolished the effect of RNS and Vt, but not that of KCl, to increase overflow of tritium and to produce renal vasoconstriction. Administration of amiloride (180 microM) enhanced the overflow of tritium but attenuated the associated renal vasoconstriction produced by RNS, Vt and KCl. Replacement of NaCl (75 mM) with equimolar concentration of LiCl enhanced the overflow of tritium elicited by RNS, Vt and KCl; the associated renal vasoconstriction remained unaltered. Administration of Ca++ channel blocker, flunarizine (2 microM), but not diltiazem (6 microM) or nifedipine (1.4 microM), inhibited RNS-, Vt- and KCl-induced overflow of tritium; nifedipine enhanced whereas diltiazem failed to alter tritium overflow elicited by these stimuli. The renal vasoconstriction produced by RNS, Vt and KCl was inhibited by diltiazem and nifedipine, as well as by flunarizine. These data suggest that RNS, Vt and KCl enhance release of the adrenergic transmitter by promoting influx of Ca++ into the nerve terminal, not by Na+-Ca++ exchange transport process, but rather through specific Ca++ channels that are distinct from those located in the vascular smooth muscle.