The Influence of the Adenosine A1-receptor on Tubuloglomerular Feedback and Renin Release

Sammanfattning: The kidneys play a vital role in the maintenance of extracellular fluid and electrolyte balance and blood pressure. Adenosine, acting through the adenosine A1-receptor (A1R), and nitric oxide have been implicated in several of the regulatory mechanisms in the kidney. The A1R has been found to be present in the renal vasculature, primarily in the afferent arterioles, and in the proximal tubules. The tubuloglomerular feedback mechanism (TGF) is an important regulator of renal vascular tone and glomerular filtration rate. The aim of these investigations was to further elucidate the role of adenosine, acting through the A1R. Investigations on adenosine’s renal effects were performed on transgenic mice lacking the A1R.TGF response, elicited by increased distal salt load, was completely abolished in the A1R knockout (A1R -/- ) mice. Basal plasma-renin levels were found to be ~2-fold higher in the A1R -/- compared to the A1R wild-type (A1R+/+) mice. However, salt intake induced inverse changes in plasma-renin levels, indicating that adenosine tonically inhibits macula densa stimulated renin release. Anesthetized and conscious A1R -/- mice, measured telemetrically, had an increased blood pressure, which could be due to the increased plasma-renin levels. Despite the high plasma-renin levels, increased urinary sodium excretion was also observed in the A1R -/- animals. Ischemia caused a decrease in renal function in both A1R+/+ and A1R -/- mice. Ischemic preconditioning protected the A1R+/+ mice from subsequent ischemic episode but had no protective effect on the A1R -/- mice.Acute extracellular volume expansion greatly attenuates TGF sensitivity, thus facilitating the elimination of excess fluid. Acute inhibition of nNOS in volume-expanded rats was found to re-establish the attenuated TGF response caused by acute extracellular volume expansion.The results show that adenosine, acting through the A1R, plays an important role in mediating TGF response and consequently, regulating renin release, blood pressure, electrolyte balance and other vital renal mechanisms.