The purpose of this study was to establish the existence of Na/H exchange in cardiac muscle and to evaluate the contribution of Na/H exchange to pHi regulation . The kinetics of pH i changes in cultured chick heart cells were monitored microfluorometrically with 6-carboxyfluorescein and correlated with Nai content changes analyzed by atomic absorption spectrophotometry ; transmembrane H+ movements were evaluated under pH stat conditions . After induction of an intracellular acid load by pretreatment with NH 4C1, a regulatory cytoplasmic alkalinization occurred with a t1/2 of 2.9 min. pHi regulation required external Na' and was concomitant with transmembrane H+ extrusion as well as a rapid rise in Nai content in an Na/H ratio of 1 :1 . Microelectrode recordings of membrane potential demonstrated directly the electroneutral character of pHi regulation . Acid-induced net Na' uptake could be either stimulated by further decreasing pHi or inhibited by decreasing pH .; Na' uptake was unaffected by tetrodotoxin (10 ug/ml), quinidine (10-3 M), DIDS (10-4 M), C16-free solution, or HC03-free solution . Amiloride (10-3 M) maximally inhibited both pHi regulation and Na' uptake ; the ID 3a for amiloride inhibition of Na* uptake was 3 I.M . Na.-dependent H+ extrusion showed halfmaximal activation at 15 mM Na.; Li+, but not K+ or choline', could substitute for Na' to support H+ extrusion. Ca.-free solution also stimulated acid-induced Na* uptake . We conclude that pHi regulation following an acid load in cardiac muscle cells is by an amiloride-sensitive, electroneutral Na/H exchange. Stimulation of Na/H exchange up to 54 pmol/cm2 .s indicates the rapidity of this exchange across cardiac cell membranes. Na/H exchange may also participate in steady state maintenance of pHi.

The neutral sphingomyelinases (N-SMases) are a group of Mg²⁺-dependent enzymes with a pH optimum in the neutral range. N-SMases catalyze the conversion of sphingomyelin to ceramide and have been found particularly enriched in brain tissue. N-SMase activity has been implicated in many physiological and pathological processes affecting the brain and nervous system. In this review, we discuss the proposed functions of N-SMase with a particular emphasis on its role in neurological disorders, such as age-related neurodegeneration, Alzheimer's disease, HIV-associated dementia, atherosclerosis, ischemia-reperfusion injury, and cancer.

No abstract

Considerable difficulty has been encountered in defining the physiological significance of sodium tracer kinetic measurements in cardiac muscle. In this study, 24Na+ efflux experiments were performed by directly monitoring tissue radioactivity during the superfusion of growth-oriented embryonic chick heart cells in tissue cultured. The cellular 24Na+ efflux from contractile preparations exhibited at least two exponential components whereas noncontractile, fibroblastlike preparations had a single efflux component similar in rate to the slower component of the contractile preparations. We concluded that the slow component represents efflux from nonmuscle cells, whereas the faster component reflects the muscle cell compartment. The mean Na+ efflux rate constants for contractile preparations (beating 150 min-1) were 3.1 and 0.35 min-1. Intracellular Na+ concentrations, as determined by isotope uptake and by flame photometry, were 18 and 16 mM for contractile and nonmuscle preparations, respectively. The steady-state, transmembrane fluxes are 98 and 5 pmol . cm-2 . s-1 for muscle and nonmuscle cells, respectively. The Na+ efflux kinetics in 10(-4) M ouabain were reduced by approximately 16% from the control value. These findings indicate that the greater part of the steady-state Na+ efflux in cultured heart cells is due to mechanisms other than the Na+-K+ pump.

To evaluate the transmembrane movement of chloride in a preparation of cardiac muscle lacking the extracellular diffusion limitations of natural specimens, intracellular chloride concentration ([Cl]i) and transmembrane ~C1 efflux have been determined in growth-oriented embryonic chick heart cells in tissue culture. Using the method of isotopic equilibrium, [Cl]i was 25.1 ± 7.3 mmol. (liter cell water) -1, comparable to the value of 24.9 ± 5.4 mmol. (liter cell water) -1 determined by coulometric titration. Two cellular 36C1 compartments were found; one exchanged with a rate constant of 0.67 + 0.12 min -x and was associated with the cardiac muscle cells; the other, attributed to the fibroblasts, exchanged with a rate constant of 0.18 ± 0.05 min -1. At 37°C, transmembrane Cl flux of cardiac muscle under steady-state conditions was 30 pmol.cm-2-s -1. In K-free, normal, or high-Ko solutions, the responses of the membrane potential to changes in external Cl concentration suggested that chloride conductance was low. These results indicate that CI transport across the myocardial cell membrane is more rapid than K transport and is largely electrically silent.