Intrarenal vascular sites of action of adenosine (Ad) and glucagon (G1)

The current study was designed to evaluate the intrarenal sites (afferent and efferent arterioles) of vascular resistance changes induced by different vasodilators. Renal blood flow (RBF) and glomerular filtration rate (GFR) were measured in pentobarbital anesthetized dogs prior to and during infusicn of acetylcholine (ACh). Ad, and G1 into the renal artery. Renal arterial pressure (RAP) was varied by means of an adjustable aortic clamp. All test agents increased RBF. However, the effects on GFR were various：GFR was increased by G1, decreased by Ad, and unchanged by ACh. The ratio of the increase in RBF at RAP of 75 mmHg to the increase in RBF at RAP of 125 mmHg was calculated for three agents. The ratio for ACh was about 0.5. However, at 75 mmHg, the vasodilation induced by G1 was almost abolished, whereas that induced by Ad was almost unaffected; the ratios for G1 and Ad were approximately 0 and 1, respectively Since it is generally accepted that afferent arteriolar resistance attains a minimum value at RAP near the lower limit of the autoregulatory range, increases in RBF at 75 mmHg should reflect changes mainly in efferent resistance. Thus, our data indicate that ACh acts on both afferent and efferent arterioles. In contrast, G1 acts mainly on the afferent arteriole, whereas Ad acts mainly on the efferent arteriole.