Bethanechol inhibition of chicken intestinal brush border Na/H exchange: Role of protein kinase C and other calcium-dependent processes

A unique Na+/H+ exchanger, analogous to NHE1, in the chicken embryonic fibroblast

We report the characterization of an Na+/H+ exchanger (NHE) in embryonic fibroblasts (SL-29 cells) of the chicken, a terrestrial vertebrate, where Na+ conservation is important. This exchanger is electroneutral, has a single Na+ binding site, and is highly sensitive to amiloride (IC50 2 microM), dimethyl amiloride (350 nM), and ethyl-isopropyl amiloride (25 nM). It is stimulated by serum, transforming growth factor-alpha, hypertonicity, and okadaic acid. Although these features make it resemble mammalian NHE1, other characteristics suggest distinct differences. First, in contrast to mammalian NHE1 it is inhibited by cAMP and shows a biphasic response to phorbol esters and a highly variable response to increased intracellular Ca2+ concentration. Second, whereas full-length human and rat NHE1 cDNA probes recognize a 4.8-kb transcript in rat tissues, they recognize only a 3.9-kb transcript in chicken tissues. An antibody against amino acids 631-746 of human NHE1 sequence fails to recognize a protein in SL-29 cells. Rat NHE2 and NHE3 probes do not recognize any transcript in chicken fibroblasts. The SL-29 exchanger differs markedly from the previously characterized chicken intestinal apical exchanger in its amiloride sensitivity and regulation by phorbol esters. These results suggest that a modified version of mammalian NHE1 is present in chicken tissues and imply that another functionally distinct Na+/H+ exchanger is expressed in aves.

An avian sodium-hydrogen exchanger

Intestinal sodium transporters, such as the Na+/H+ exchanger (NHE) are important for Na+ conservation in land birds. In mammals, at least five isoforms of the exchanger, NHEs 1-5, have been cloned, with NHE-1 occurring in epithelial basolateral and nonepithelial cell membranes and NHE-3 being restricted to epithelial apical/brush border membranes. We had demonstrated earlier that chicken intestinal brush border membranes possess NHE activity that functionally resembles mammalian NHE-3. In this study, we used mammalian NHE-1 and NHE-3 probes to examine if chicken enterocytes possess these transporters. Antisera against rat NHE-3 recognized a 97 kDa protein in chicken intestinal brush border membrane, while a NHE-3 cDNA probe failed to recognize any transcript. A NHE-1 antibody failed to recognize any protein in brush border or basolateral membrane, while a NHE-1 cDNA probe recognized a 3.9 kb transcript. Thus, there is more than one NHE isoform in chicken intestine, and our results suggest a novel avian NHE family.

PKC activators stimulate H+ conductance in chicken enterocytes

Chicken enterocytes present a H+-conducting pathway involved in the recovery of intracellular pH (pHi) from an acid load. In the current study we have tested the effect of protein kinase C (PKC) activators on the rate of proton efflux through the H+-conducting pathway. The rate of proton efflux was increased by the addition of 1,2-dioctanoyl-rac-glycerol (DOG) or phorbol 12-myristate 13-acetate (PMA), but it was not affected by the addition of the inactive phorbol ester analogue, 4alpha-phorbol 12, 13-didecanoate. DOG stimulated the process in a dose-dependent manner with a half-maximal effect at 45 microM. Staurosporine and Zn2+ prevented the DOG-dependent increase in the rate of proton efflux. The rate of proton efflux was affected by the pH, and DOG shifted this relationship upward and to the right. These results suggest that the proton-conducting pathway is regulated by PKC.

Phorbol dibutyrate-specific protein phosphorylation in brush border membranes of chicken enterocytes

We have demonstrated that phorbol esters such as phorbol dibutyrate (PhE) transiently inhibit Na/H exchange both in intact avian enterocytes and in brush border membrane (BBM) vesicles prepared from enterocytes treated with PhE (Chang et al., 1991, Am. J. Physiol. 260: C1264-C1272). Maximal inhibition occurs at 90 sec and values return to baseline by 15 min. In this study we examined if PhE causes changes in BBM protein phosphorylation by two methods: 1) in situ phosphorylation in which intact cells prelabeled with 32Pi were treated with PhE; 2) in vitro phosphorylation in which BBM, isolated from untreated and PhE-treated enterocytes, were exposed to gamma 32P-ATP. In situ phosphorylation studies showed that, at 90 sec, PhE increases the phosphorylation of BBM proteins of M(r) (pI): 150 (6.5), 89 (approximately 6.2), and 48 (approximately 6.1) kDa which declined to control values at 15 min, suggesting that these may be transport-related substrates. These labeled substrates were recovered in the detergent-insoluble fraction after extraction with 0.1% Triton X-100 overnight. Transient phosphorylation of a number of proteins was also observed when BBM prepared from control or PhE-treated cells were incubated with gamma 32P-ATP +/- 10 nM PhE, phosphatidyl serine, Ca2+, and/or exogenous protein kinase C (PKC). The in vitro phosphoproteins included both Triton-soluble and Triton-insoluble proteins. However, none of these proteins labeled in vitro coincided with those labeled in situ. The decline in phosphorylation with time can be accounted for by phosphatase action as these BBM possess a Ca-dependent phosphatase. In summary, we have demonstrated that the BBM possess PKC-specific substrates which can be visualized by in situ and in vitro phosphorylation. Treatment of intact enterocytes with PhE results in the phosphorylation of three detergent-insoluble proteins with a time course similar to that of PhE inhibition of Na/H transport.

Rectal epithelial expression of protein kinase A phosphorylation of cystic fibrosis transmembrane conductance regulator

BACKGROUND/AIMS

Human rectal epithelium in cystic fibrosis (CF) shows impaired ion transport in response to theophylline or bethanechol, although it possesses regulatory subunits of adenosine cyclic 3',5'-monophosphate (cAMP)-dependent protein kinase (protein kinase A). Protein kinase A-specific phosphorylation of CF transmembrane conductance regulator (CFTR) in rectal tissues of control and CF volunteers was examined in this study.

METHODS

CFTR was evaluated using a polyclonal antiserum (pre-NBF) raised against a peptide corresponding to residues 415-427 of CFTR. Microsomal membranes from normal and CF rectal mucosa and from T-84 cells were incubated with [gamma 32P]-adenosine triphosphate +/- protein kinase A and subjected to immunoblotting with pre-NBF and autoradiography.

RESULTS

Pre-NBF recognized a single band of 180 kilodaltons. Protein kinase A altered phosphorylation of this 180-kilodalton band 1.4-, 2.2- and 0.9-fold in T-84, normal, and CF rectal membranes, respectively. Catalytic activities of protein kinase A, Ca2+ calmodulin protein kinase, or protein kinase C in control and CF tissues were similar.

CONCLUSIONS

cAMP and Ca(2+)-signaling pathways are normal up to the kinases in CF rectal mucosa. Our results suggest differences in CFTR phosphorylation in normal and CF rectal mucosal membranes.

Na+/H+ exchangers, NHE-1 and NHE-3, of rat intestine. Expression and localization

Na-H exchange (NHE) is one of the major non-nutritive Na absorptive pathways of the intestine and kidney. Of the four NHE isoforms that have been cloned, only one, NHE-3, appears to be epithelial specific. We have examined the regional and cellular expression of NHE-3 in the rat intestine. NHE-3 message in the small intestine was more abundant in the villus fractions of the small intestine than in the crypts. Analysis of NHE-3 mRNA distribution in the gut by in situ hybridization demonstrated epithelial cell specificity, as well as expression preferential to villus cells. NHE-1 message, in contrast, was ubiquitous, with slightly greater expression exhibited in the differentiating crypt and lower villus cells of the small intestine. Isoform-specific NHE-3 fusion protein antibody identified a 97-kD membrane protein in the upper villus cells of the small intestine, which was exclusively localized in the apical membrane. In contrast, antibody previously developed against the COOH-terminal region of human NHE-1 (McSwine, R. L., G. Babnigg, M. W. Musch, E. B. Chang, and M. L. Villereal, manuscript submitted for publication) identified a 110-kD basolateral membrane protein. These data suggest that unlike NHE-1, which probably serves a "housekeeping" function, NHE-3 may be involved in vectorial Na transport by the intestine.

Muscarinic stimulation of gallbladder epithelium. II. Fluid transport, cell volume, and ion permeabilities

Activation of muscarinic receptors in the fluid-absorptive epithelium of the Necturus gallbladder elevates cytosolic Ca2+ concentration, transiently hyperpolarizes the cell membrane voltages, and decreases the apparent fractional resistance of the apical membrane [G. A. Altenberg, M. Subramanyam, J. S. Bergmann, K. M. Johnson, and L. Reuss. Am. J. Physiol. 265 (Cell Physiol. 34): C1604-C1612, 1993]. In these studies, we show that at the peak of the hyperpolarization both apical and basolateral membrane resistances (Ra and Rb, respectively) decreased, but in 2-3 min Ra returned to control values while Rb rose to a level approximately 60% higher than control. The acetylcholine (ACh)-induced decrease in Ra is caused by activation of apical membrane maxi K+ channels secondary to elevation of cytosolic Ca2+ concentration. The increase in Rb is due to decreases in K+ and Cl- conductances. ACh had no effects on cell KCl content or water volume, although K+ conductance transiently increased. These results can be explained by the changes in basolateral membrane conductances. ACh did not alter fluid absorption. In conclusion, ACh has complex time-dependent effects on K+ and Cl- electrodiffusive permeabilities without measurable changes in cell volume or in the rate of transepithelial fluid transport.

Muscarinic stimulation of gallbladder epithelium. I. Electrophysiology and signaling mechanisms

To understand the effects of acetylcholine (ACh) on fluid-absorbing epithelia, we carried out experiments on Necturus gallbladder epithelium. Binding studies with 1-quinuclidinyl[phenyl-4(N)-3H]benzilate (QNB) demonstrated that Necturus gallbladder epithelial cells express high-affinity muscarinic receptors. The effects of ACh and carbachol were exerted from the basolateral surface and consisted of a transient hyperpolarization of both cell membranes and a concomitant decrease in the apparent fractional resistance of the apical membrane. Atropine blocked both effects. ACh also elicited transient elevations of inositol 1,4,5-trisphosphate and intracellular free calcium ([Ca2+]i) levels, the latter by both release from intracellular stores and basolateral influx. The phospholipase C antagonist U-73122 inhibited the effects of ACh, whereas inhibition of prostaglandin and guanosine 3',5'-cyclic monophosphate synthesis with indomethacin or methylene blue, respectively, had no effect. In conclusion, Necturus gallbladder epithelium expresses muscarinic receptors in the basolateral membrane. Receptor activation stimulates phospholipase C and elevates cellular levels of inositol 1,4,5-trisphosphate and [Ca2+]i. The elevation in [Ca2+]i activates K+ channels but apparently not Cl- channels.