Evidence for cholinergic innervation in dog renal tissue

Nicotinic acetylcholine receptor expression and regulation in the rat kidney after ischemia-reperfusion injury

The cholinergic anti-inflammatory pathway is a mechanism whereby local inflammation is modulated by the brain via the vagus nerve and nicotinic acetylcholine receptors (nAChRs). The nAChR family are ligand-gated ion channels that consist of many different subtypes formed by the specific assembly of five polypeptide subunits including alpha1-10, beta1-4, gamma, delta, and epsilon. The alpha7 receptor (alpha7nAChR) mediates the anti-inflammatory effects of cholinergic stimulation. We recently demonstrated that cholinergic agonists attenuate renal ischemia-reperfusion (I/R) injury in rats. We also showed that tubular epithelial cells express functional nAChRs in vitro. The current studies report the expression, localization, and regulation of the alpha7nAChR in the rat kidney after I/R injury. We also examined, in this model, potential interactions between cholinergic stimulation and the STAT3 pathway, a key signaling cascade that has been linked to alpha7nAChR activation. RT-PCR and immunohistochemistry showed constitutive expression of many nAChR subunits. Immunohistochemistry localized basal alpha7nAChR expression to the endothelium of cortical peritubular capillaries, and its distribution was upregulated after I/R injury. Western blotting also showed an increase in alpha7nAChR subunit protein after renal I/R injury. Interestingly, pretreatment with nicotine, which improves the outcome after renal I/R injury, reduced the alpha7nAChR protein after I/R injury. Finally, we found that I/R injury stimulated the STAT3 pathway, whereas pretreatment with nicotine downregulated its activation. These results suggest that the alpha7nAChR plays an important role in the pathophysiology of renal I/R injury.

SFKs, Ras, and the classic MAPK pathway couple muscarinic receptor activation to increased Na-HCO(3) cotransport activity in renal epithelial cells

Cholinergic agents are known to affect the epithelial transport of H2O and electrolytes in the kidney. In proximal tubule cells, cholinergic agonists increase basolateral Na-HCO(3) cotransport activity via M(1) muscarinic receptor activation. The signaling intermediates that couple these G protein-coupled receptors to cotransporter activation, however, are not well defined. We therefore sought to identify distal effectors of muscarinic receptor activation that contribute to increased NBC activity in cultured proximal tubule cells. As demonstrated previously for acute CO2-regulated cotransport activity, we found that inhibitors of Src family kinases (SFKs) or the classic mitogen-activated protein kinase (MAPK) pathway prevented the stimulation of NBC activity by carbachol. The ability of carbachol to activate Src, as well as the proximal (Raf) and distal [extracellular signal-regulated kinases 1 and 2 (ERK1/2)] elements of the classic MAPK module, was compatible with these findings. Cholinergic stimulation of ERK1/2 activity was also completely prevented by overexpression of a dominant negative mutant of Ras (N17-Ras). Taken together, these findings suggest a requirement for the sequential activation of SFKs, Ras, and the classic MAPK pathway [Raf-->MAPK/ERK kinase (MEK)-->ERK]. These findings provide important insights into the molecular mechanisms underlying cholinergic regulation of NBC activity in renal epithelial cells. They also suggest a specific mechanism whereby cholinergic stimulation of the kidney can contribute to pH homeostasis.

Inhibition of amiloride-sensitive apical Na+ conductance by acetylcholine in rabbit cortical collecting duct perfused in vitro

We examined effects of acetylcholine (ACh) on the electrical parameters and intracellular Ca2+ concentration ([Ca2+]i) in the isolated rabbit cortical collecting duct (CCD) perfused in vitro using the conventional microelectrode technique and microscopic fluorescence spectrophotometry. ACh (10(-8) to 10(-5) M) in the bath caused a positive deflection of the transepithelial voltage (VT) and an increase in [Ca2+]i. Carbachol also showed similar but smaller effects. The effects of ACh were antagonized by muscarinic receptor antagonists. ACh at 10(-6) M hyperpolarized the apical membrane voltage and increased the fractional resistance of the apical membrane of the collecting duct cells accompanied by a positive deflection of VT and an increase in transepithelial resistance, whereas it did not affect these parameters in the beta-intercalated cells. In the presence of 10(-5) M amiloride in the lumen, the effects of ACh were almost completely abolished. The ACh-induced increase in [Ca2+]i is accounted for by the release of Ca2+ from intracellular store and Ca2+ entry from the bath. In the absence of Ca2+ in the bath, the ACh-induced changes in electrophysiological parameters were significantly smaller than those observed in the presence of Ca2+. Both phorbol-12-myristate-13-acetate (PMA) and phorbol-12,13-dibutylate (PDBu), activators of protein kinase C (PKC), also inhibited the apical Na+ conductance. In the presence of PMA or PDBu in the bath, ACh did not show further inhibitory effect. 1-(5-Isoquinolinylsulfonyl)-2-methylpiperazine, an inhibitor of PKC, partially attenuated the effect of ACh. These observations indicate that ACh inhibits the apical Na+ conductance partly by both increasing [Ca2+]i and activating PKC. Such an action of ACh may partially explain its natriuretic effect.

Acetylcholine and kinin augmentation of Cl- secretion stimulated by prostaglandin in a canine renal epithelial cell line

1. The actions of kinins and of acetylcholine upon transepithelial ion transport in a renal-derived cultured epithelium (Madin-Darby canine kidney cells, MDCK) have been investigated. 2. In voltage-clamped epithelial layers mounted in Ussing chambers and with prior stimulation of inward short-circuit current (SCC) by 5 or 10 microM-prostaglandin E1 (PGE1), both bradykinin (1 microM) and acetylcholine (0.1 mM) stimulate an additional, but transient, inward SCC. In the absence of PGE1 minimal effects of both bradykinin and acetylcholine upon SCC are observed. The SCC response to bradykinin and acetylcholine are attenuated with prior stimulation by 10 microM-adrenaline. 3. Measurements of bradykinin and acetylcholine-stimulated inward SCC with cation and anion replacement of the bathing media and the use of the Cl channel blocker 5-nitro-2(3-phenylpropylamino)-benzoic acid (NPPB) together with bumetanide to inhibit Na(+)-K(+)-Cl- 'co-transport', are consistent with the bradykinin- and acetylcholine-stimulated SCC being the result of basal to apical Cl- secretion. 4. Bradykinin (1 microM) is capable of stimulation of inward SCC from both epithelial surfaces, whilst acetylcholine is only effective from the basolateral surface. Kallidin (lys-bradykinin) was similar in effect to bradykinin from both epithelial surfaces whereas bradykinin (1-8) was ineffective, suggesting that B2 bradykinin receptors mediate the effect of bradykinin upon SCC. Dose-response relationships show that the response to kallidin and bradykinin was of higher sensitivity for additions to the apical cell aspects. 5. The data are discussed in relation to a model for epithelial Cl- secretion, and to the mechanism of natriuresis observed with kinins and acetylcholine in vivo.