Activation of renal afferent pathways following furosemide treatment II. Effect of angiotensin blockade

The goal here and in the accompanying paper was to evaluate the two pathway s used by the kidney to provide information to the central nervous system ( CNS); e.g., the indirect, hormonal route via activation of the renin-angiot ensin system and the direct pathway via activation of sympathetic afferents in the caudal thoracic spinal cord. Here, three experiments were designed to evaluate the actions of angiotensin elicited by subcutaneous injection o f furosemide on neural activation of the CNS. The number of neurons immunoc ytochemically staining for the protein product (Fos) of the c-fos gene was used as an index of neuronal activation. In the first experiment, furosemid e injection was preceded by treatment with a dose of Captopril, CAP, (an an giotensin-converting enzyme (ACE) inhibitor) that blocks the peripheral but not the central formation of angiotensin II. In the second experiment, fur osemide injection was preceded by treatment with a higher dose of CAP; this dosage blocks the peripheral and central formation of angiotensin II. In t he third experiment, furosemide injection was preceded by treatment with Lo sartan, a competitive receptor antagonist of type I angiotensin II receptor s at a dose that would block central and peripheral angiotensin receptors. Control animals in each experiment received injections of vehicle (sterile isotonic saline) instead of furosemide. In each experiment, rats were sacri ficed 1.75 h following furosemide or saline injection by transcardial perfu sion and tissues were immunocytochemically processed for demonstration of F os antigen. Rats receiving furosemide plus the low CAP dose showed more Fos -positive cells than control rats in the subfornical organ (SFO), organum v asculosum lamina terminalis (OVLT), supraoptic nucleus (SON), magnocellular region of the paraventricular nucleus, nucleus of the solitary tract (NTS) , and caudal thoracic/rostral lumbar spinal cord dorsal hem. Rats receiving furosemide plus Losartan or furosemide plus the higher CAP dose did not sh ow increased Fos immunoreactivity in any of the abovementioned structures r elative to their respective control animals. We conclude that the receptor- mediated action of angiotensin II is in some way involved in the activation of the pathway that occurs in the SFO, OVLT, SON, and magnocellular region of the paraventricular nucleus (PVN) in response to furosemide treatment. It is possible that the furosemide-induced activation in the SON and PVN is not due to direct actions of angiotensin II on angiotensin receptors in th ose structures, but instead occurs synaptically as a result of inputs from the SFO and OVLT, which have themselves been activated directly by angioten sin II. In the accompanying paper, furosemide-induced activation in the NTS and caudal thoracic spinal cord is abolished by prior bilateral renal dene rvation, meaning that these neurons are likely part of a renal afferent pat hway. Here, these structures did not elaborate Fos in animals injected with furosemide plus the high CAP dose or furosemide plus Losartan. Thus, the p resent results also suggest that the central blockade of the formation of a ngiotensin II or blockade of the actions of angiotensin II prevents in some way the activation of the renal afferent pathway mediated by the renal ner ves (the direct pathway) in response to the actions of furosemide. Therefor e, these results suggest that central angiotensin II is somehow involved in "priming" or increasing the sensitivity of the direct renal afferent pathw ay. Taken together with the accompanying paper, our results indicate that inter ruption of the direct pathway via renal denervation did not interfere with the elaboration of Fos in the lamina terminalis; in contrast, modification of the humoral renal afferent pathway can affect the sensitivity of the dir ect pathway. These results may have important implications for pathophysiol ogical changes associated with fluid balance disorders including renal hype rtension. (C) 2000 Elsevier Science B.V. All rights reserved.