Expression of adrenergic and cholinergic receptors in murine renal intercalated cells.

Neurons influence renal function and help to regulate fluid homeostasis, blood pressure and ion excretion. Intercalated cells (ICCs) are distributed throughout the renal collecting ducts and help regulate acid/base equilibration. Because ICCs are located among principal cells, it has been difficult to determine the effects that efferent nerve fibers have on this cell population. In this study, we examined the expression of neurotransmitter receptors on the murine renal epithelial M-1 cell line. We found that M-1 cells express a2 and b2 adrenergic receptor mRNA and the b2 receptor protein. Further, b2 receptor-positive cells in the murine cortical collecting ducts also express AQP6, indicating that these cells are ICCs. M-1 cells were found to express m1, m4 and m5 muscarinic receptor mRNAs and the m1 receptor protein. Cells in the collecting ducts also express the m1 receptor protein, and some m1-positive cells express AQP6. Acetylcholinesterase was detected in cortical collecting duct cells. Interestingly, acetylcholinesterase-positive cells neighbored AQP6-positive cells, suggesting that principal cells may regulate the availability of acetylcholine. In conclusion, our data suggest that ICCs in murine renal collecting ducts may be regulated by the adrenergic and cholinergic systems.

Sympathetic nerve varicosities in close apposition to basolateral membranes of collecting duct epithelial cells of rat kidney.

BACKGROUND: There are reports of sympathetic innervation of the nephron and of P2 purinergic receptors on epithelial cells. Since ATP is a cotransmitter with noradrenaline in sympathetic nerves, the objective of the present study was to re-investigate basolateral innervation of rat renal collecting duct epithelial cells by sympathetic nerves in the context of recent data on the effects of ATP on this nephron segment. METHODS: Kidney sections were processed for electron immunocytochemistry, using tyrosine hydroxylase rabbit polyclonal antibody, with a second layer of biotinylated donkey anti-rabbit antibody and finally extravidin-horseradish peroxidase. Immunoreactivity was visualised with 3,3'-diaminobenzidine and examined with a Philips CM120 transmission electron microscope. RESULTS: Electron microscopic evidence is presented for close apposition of sympathetic nerve varicosities immunolabelled with tyrosine hydroxylase to principal and intercalated type epithelial cells of the collecting duct of the cortical region. CONCLUSIONS: It is suggested that ATP is released as a cotransmitter from sympathetic nerve varicosities to act on basolateral P2 purinoceptors to influence sodium and water (and potentially acid-base) transport, in conjunction with the known (typically inhibitory) actions of autocrine and/or paracrine release of ATP from collecting duct epithelial cells acting via luminal P2 receptors. It is suggested that while luminal responses may dominate under normal physiological conditions, in pathophysiological states, such as stress and dehydration, sympathetic nerves might also be involved in modulating collecting duct fluid and electrolyte transport.

Neural regulation of renal tubular sodium reabsorption and renin secretion.

The entire mammalian nephron, including the juxtaglomerular apparatus, receives an exclusive noradrenergic innervation. Renal tubular alpha 1 adrenoceptors mediate the alterations in tubular segmental sodium, chloride, and water reabsorption that occur in response to direct or reflex changes in efferent renal sympathetic nerve activity. Specific tubular segments so identified are the proximal convoluted tubule, the loop of Henle (thick ascending limb), and the collecting duct. Alterations in efferent renal sympathetic nerve activity represent an important physiological contribution to the overall role of the kidney in the regulation of external sodium balance in conscious animals during both dietary sodium restriction and acute and chronic increases in total-body sodium. Progressively more intense activation of the renal nerves recruits a series of adrenergically mediated influences on renin secretion that are additive, ranging from subtle (modulation of nonneural mechanisms without directly causing renin secretion) to marked (renal vasoconstriction, antinatriuresis, high renin secretion rates). Juxtaglomerular granular cell beta 1 adrenoceptors mediate renin secretion responses to frequencies of renal nerve stimulation that do not cause renal vasoconstriction; at higher frequencies of renal nerve stimulation where renal vasoconstriction is present, renal vascular alpha 1 adrenoceptors mediate a portion of the renin secretion response.

Distribution of alpha-adrenergic receptors in the rabbit nephron.

In order to determine the distribution of alpha-adrenergic receptors within a nephron, a method was used to determine the specific binding of [3H]-prazosin to isolated fragments of rabbit renal tubules. When 10(-8) moles/liter [3H]-prazosin was incubated with proximal convoluted tubules, 61.4% of total binding was accounted for specific binding. The [3H]-prazosin binding to the proximal convoluted tubule was a linear function of the tubular length and reversible. It was inhibited with 10(-4) moles/liter phenoxybenzamine by about 60%, and with 10(-4) moles/liter norepinephrine by about 25%, but not with either atenolol or isoproterenol. No specific binding of [3H]-prazosin was observed in the proximal straight tubule and in the cortical collecting tubule. These data are in good agreement with the view that alpha-1 adrenergic receptors are mainly distributed in the proximal convoluted tubules.

Electron-microscopic study of innervation of smooth muscle cells surrounding collecting tubules of the fish kidney.

The fine structure of the collecting tubules of the trout and killifish kidney was studied. These tubules are surrounded by layers of smooth muscle cells which are commonly innervated. The nerve terminals contain synaptic vesicles and, occasionally, a few dense-cored granules as well. Capillaries occur in the connective tissue space between these smooth muscle cells and the collecting tubule. Epithelial cells of the collecting tubules contain abundant mitochondria and a well developed membrane system displaying parallel arrays, and were considered to be actively involved in the transport of materials. In the trout, the collecting tubules contain peculiar cells in addition to regular tubule cells. The fine structure of these peculiar cells is highly reminiscent of that of gill chloride cells. The significance of these findings may be summarized as follows: If the smooth muscles around the collecting tubule contract under neural influence, intratubular pressure may be increased and, thus affect glomerular filtration rate. The contraction of these muscles may also cause the collapse of peritubular capillaries, affecting the transport activity of tubule cells.