Quantitative analysis of the high-affinity binding sites for [3H]ouabain in the rat vas deferens and their immunological identification as the alpha 2 isoform of Na+/K(+)-ATPase

Mechanisms of adaptive supersensitivity in vas deferens

Adaptive supersensitivity is a phenomenon characteristic of excitable tissues and discloses as a compensatory adjustment of tissue's response to unrelated stimulatory endogenous and exogenous substances after chronic interruption of excitatory neurotransmission. The mechanisms underlying such higher postjunctional sensitivity have been postulated for a variety of cell types. In smooth muscles, especially the vas deferens with its rich sympathetic innervation, the mechanisms responsible for supersensitivity are partly understood and appear to be different from one species to another. The present review provides a general understanding of adaptive supersensitivity and emphasizes early and recent information about the putative mechanisms involved in this phenomenon in rodent vas deferens.

Time-dependent up-regulation of Ca(2+) channels in vas deferens of newborn rats fed with breast milk of mothers under treatment with nifedipine

Our aim was to check for calcium channel maturation and regulation on newborn rats during breastfeeding by mothers treated with the L-type calcium channel blocker nifedipine. Contractions by KCl and radioligand binding techniques were used to verify if Ca(2+) channels are modified in rat vas deferens of 40-day old litters that were breastfed by mothers injected daily with nifedipine during nursery. Injections were applied in the beginning (1st until 8th day), middle (9th until 16th day), or end (17th until 24th day) of nursery, to verify the period of highest susceptibility of newborn to nifedipine receptor regulation. Contractile responses revealed that only after the middle period of treatment of mothers the maximal effects (E(max)) induced in pups by KCl were increased by about 35%, without changes of apparent affinity (pD(2)). Additionally, binding studies with [(3)H] Isradipine in cell membrane preparations showed a greater density (B(max)) of Ca(2+) channels by about 55%, without changes of affinity (K(d)). Changes were not detected after treatment of mothers in the beginning or end of breastfeeding. In addition, in vas deferens of 60-day old litters, the E(max) returned to control values, showing that changes were not persistent. Moreover, body and vas deferens weights and blood testosterone of newborn were never changed. The histology of mammary gland was similar for treated and control mothers, suggesting a stable milk production. It is concluded that nifedipine treatment of mothers, if made during the 9th to 16th day of lactation, produced a short lasting reversible up-regulation of L-type Ca(2+) channels.

Na,K-ATPase Subunit Heterogeneity as a Mechanism for Tissue-Specific Ion Regulation

The Na,K-ATPase comprises a family of isozymes that catalyze the active transport of cytoplasmic Na+ for extracellular K+ at the plasma membrane of cells. Isozyme diversity for the Na,K-ATPase results from the association of different molecular forms of the alpha (alpha1, alpha2, alpha3, and alpha4) and beta (beta1, beta2, and beta3) subunits that constitute the enzyme. The various isozymes are characterized by unique enzymatic properties and a highly regulated pattern of expression that depends on cell type, developmental stage, and hormonal stimulation. The molecular complexity of the Na,K-ATPase goes beyond its alpha and beta isoforms and, in certain tissues, other accessory proteins associate with the enzyme. These small membrane-bound polypeptides, known as the FXYD proteins, modulate the kinetic characteristics of the Na,K-ATPase. The experimental evidence available suggests that the molecular and functional heterogeneity of the Na,K-ATPase is a physiologically relevant event that serves the specialized functions of cells. This article focuses on the functional properties, regulation, and the biological relevance of the Na,K-ATPase isozymes as a mechanism for the tissue-specific control of Na+ and K+ homeostasis.

New molecular determinants controlling the accessibility of ouabain to its binding site in human Na,K-ATPase alpha isoforms

Inhibition of Na,K-ATPase alpha2 isoforms in the human heart is supposed to be involved in the inotropic effect of cardiac glycosides, whereas inhibition of alpha1 isoforms may be responsible for their toxic effects. Human Na,K-ATPase alpha1 and alpha2 isoforms exhibit a high ouabain affinity but significantly differ in the ouabain association and dissociation rates. To identify the structural determinants that are involved in these differences, we have prepared chimeras between human alpha1 and alpha2 isoforms and alpha2 mutants in which nonconserved amino acids were exchanged with those of the alpha1 isoform, expressed these constructs in Xenopus laevis oocytes, and measured their ouabain binding kinetics. Our results show that replacement of Met119 and Ser124 in the M1-M2 extracellular loop of the alpha2 isoform by the corresponding Thr119 and Gln124 of the alpha1 isoform shifts both the fast ouabain association and dissociation rates of the alpha2 isoform to the slow ouabain binding kinetics of the alpha1 isoform. The amino acids at position 119 and 124 cooperate with the M7-M8 hairpin and are also responsible for the small differences in the ouabain affinity of the ouabain-sensitive alpha1 and alpha2 isoforms. Thus, we have identified new structural determinants in the Na,K-ATPase alpha-subunit that are involved in ouabain binding and probably control, in an alpha isoform-specific way, the access and release of ouabain to and from its binding site.

Role of noradrenaline on the expression of the Na+/K+-ATPase alpha2 isoform and the contractility of cultured rat vas deferens

Rat vasa deferentia were cultured for 3 days in Dulbecco's modified Eagle's medium in the absence or presence of 1 microM noradrenaline (NA) to investigate if the lack of NA release is the key factor to explain the selective reduction of the Na(+)/K(+)-ATPase alpha(2) isoform previously observed after in vivo denervation of this organ (Quintas et al., Biochem Pharmacol 2000;60:741-7). The lack of effects of the indirect sympathomimetic tyramine and the neuronal amine uptake blocker cocaine on NA curves indicated that cultured organs were denervated completely. Organ culture induced supersensitivity, expressed as a 6.3-fold increase of pD(2) and a 42% elevation of maximal contraction for NA but not for Ba(2+). Western blotting indicated that the level of the alpha(1) isoform of Na(+)/K(+)-ATPase was unchanged after organ culture, but the alpha(2) isoform was down-regulated drastically to levels that were barely detectable. The addition of NA to the culture medium did not prevent the reduction of alpha(2) expression although it did impede NA supersensitivity (in fact a 4-fold decrease of pD(2) and a 32% reduction of maximal response were observed after incubation in the presence of NA). A striking reduction of L-type Ca(2+) channel expression also was observed, indicated by an 85% decrease of [3H]isradipine binding sites. These data suggest that NA is a trophic factor relevant to the control of muscle contraction, mediated by alpha(1)-adrenoceptors, but not to the expression of either Na(+)/K(+)-ATPase or the L-type Ca(2+) channel.

Up-regulation of Ca(2+) channels in vas deferens after chronic treatment of newborn rats with nifedipine

Radioligand binding and contraction techniques were used to verify if L-type Ca(2+) channels are modified in rat vas deferens after treatment with the blocker nifedipine (15 microg), injected at 7, 14, 21 and 28 days after birth. Vas deferens tissue was used 10, 30 and 90 days after the last injection, to verify if modifications are persistent. Binding studies with cell membranes, using [(3)H]isradipine, showed an increase of the density (B(max)) of Ca(2+) channels by more than 60%, after 10 and 30 days, without changes of affinity (K(d)). Maximal contractions (E(max)) of KCl, were increased by 106% and 37%, respectively, after 10 and 30 days, without changes of apparent affinity (pD(2)). After 90 days, the values of B(max), K(d), E(max) and pD(2) were not different from the controls. Differences were also not found for rats injected when adult. It is concluded that treatment of newborn, but not of adult, rats with nifedipine produced a long-lasting, though reversible, up-regulation of L-type Ca(2+) channels.

Influence of development on Na(+)/K(+)-ATPase expression: isoform- and tissue-dependency

The four isoforms of the catalytic subunit of Na(+)/K(+)-ATPase identified in rats differ in their affinities for ions and ouabain. Moreover, its expression is tissue-specific, developmentally and hormonally regulated. The aim of the present work was to evaluate the influence of age on the ratio and density of these isoforms in crude membrane preparations from rat brain hemispheres, brainstem, heart ventricles and kidneys. In all tissues investigated, Na(+)/K(+)-ATPase activity was higher in adults than in neonates but brain tissues presented the most remarkable differences. In these tissues, ouabain inhibition curves for Na(+)/K(+)-ATPase activity revealed the presence of two processes with different sensitivities to ouabain. An increase of approximately sixfold in the expression of the high affinity isoforms was observed between newborn and adult rats. In contrast, the low affinity isoform increased only approximately twofold in brainstem whereas it increased ninefold in brain hemispheres. Unlike brain tissues, a decrease (almost fourfold) in the number of high affinity ouabain binding sites was observed during ontogenesis of the heart. Although limited by the inability to resolve alpha(2) and alpha(3) isoforms, present data indicate that the influence of development on the expression of Na(+)/K(+)-ATPase depends not only on the isoform, but also on the tissue where the enzyme is expressed.

Down-regulation of Na(+)/K(+)-ATPase alpha(2) isoform in denervated rat vas deferens

In the rat vas deferens, an organ richly innervated by peripheral sympathetic neurons, we have demonstrated recently the expression of alpha(1) and alpha(2), but not alpha(3) isoforms of the alpha subunit of Na(+)/K(+)-ATPase (EC 3.6.1.37), a membrane-bound enzyme of vital function for living cells (Noël et al., Biochem Pharmacol 55: 1531-1535, 1998). In the present work, we characterized, qualitatively and quantitatively, Na(+)/K(+)-ATPase alpha isoforms in denervated rat vasa deferentia. [(3)H]Ouabain binding at concentrations defined for high-affinity isoforms (alpha(2) and/or alpha(3)) detected only one class of specific binding sites in control (C) and denervated (D) vas deferens. Although the dissociation constant was similar for both groups [K(d) = 138 +/- 14 nM (C) and 125 +/- 8 nM (D)], a marked decrease in density was observed after denervation [716 +/- 81 fmol.mg protein(-1) (C) and 445 +/- 34 fmol.mg protein(-1) (D), P < 0.05]. In addition, western blotting revealed that denervated vasa deferentia produce the alpha(1) and alpha(2) isoforms but not alpha(3), just as we reported for the controls previously (Noël et al., Biochem Pharmacol 55: 1531-1535, 1998). Densitometric analysis showed a decrease of the alpha(2) isoform by about 40% in denervated organs, in very good agreement with what was shown with the [(3)H]ouabain binding technique, but no significant change in alpha(1) isoform density. Truncated alpha(1) (alpha(1)T), an isoform suggested to exist in the guinea pig vas deferens, was not detected. Altogether, our results demonstrated that Na(+)/K(+)-ATPase alpha(2) is down-regulated after sympathetic denervation of the rat vas deferens.