CALCIUM ACTIVATION MECHANISMS IN THE RENAL MICROVASCULAR RESPONSE TO EXTRACELLULAR ATP

Previous studies have suggested a paracrine role for extracellular ATP in the regulation of afferent arteriolar tone. The current study was conducted to determine the dependence of this response on calcium entr y mechanisms. Experiments were performed in vitro using the blood-perf used juxtamedullary nephron technique combined with video microscopy. The afferent arteriolar response to alpha,beta>-methylene ATP was dete rmined before and after treatment with the calcium channel blockers, d iltiazem or felodipine. alpha,beta-Methylene ATP was used to obviate c oncerns over responses being elicited by ATP or by ATP hydrolysis prod ucts such as adenosine. Previous studies have shown that afferent arte riolar responses to alpha,beta-methylene ATP are comparable to those e licited by ATP. alpha,beta-Methylene ATP (1.0 mu M) induced a rapid in itial afferent vasoconstriction of 72.5 +/- 10.6%, which partially rec overed to a stable diameter 11.3 +/- 1.7% smaller than control (P < 0. 01 vs. control). Afferent diameter returned to control diameter on rem oval of ATP from the bath. Diltiazem or felodipine treatment significa ntly increased afferent diameter by 5.6 +/- 2.3 and 16.4 +/- 4.6%, res pectively (P < 0.05). In the presence of either diltiazem or felodipin e, the initial vasoconstriction to alpha,beta-methylene ATP was attenu ated, and the sustained vasoconstriction was completely blocked. Remov al of calcium from the extracellular medium completely abolished both the initial and sustained vasoconstrictor response. Returning the extr acellular calcium concentration to the physiological range completely restored the biphasic vasoconstrictor response to alpha,beta-methylene ATP. These data suggest that both the initial and the sustained phase s of the ATP-mediated juxtamedullary afferent arteriolar vasoconstrict ion are dependent on the influx of extracellular calcium and that the sustained constriction is dependent primarily on calcium entry via vol tage-gated L-type calcium channels.