Intracellular Cl- dependence of Na-H exchange in barnacle muscle fibers under normotonic and hypertonic conditions.
We previously showed that shrinking a barnacle muscle fiber (BMF) in a hypertonic solution (1,600 mosM/kg) stimulates an amiloride-sensitive Na-H exchanger. This activation is mediated by a G protein and requires intracellular Cl-. The purpose of the present study was to determine (a) whether Cl- plays a role in the activation of Na-H exchange under normotonic conditions (975 mosM/kg), (b) the dose dependence of [Cl-]i for activation of the exchanger under both normo- and hypertonic conditions, and (c) the relative order of the Cl-- and G-protein-dependent steps. We acid loaded BMFs by internally dialyzing them with a pH-6.5 dialysis fluid containing no Na+ and 0-194 mM Cl-. The artificial seawater bathing the BMF initially contained no Na+. After dialysis was halted, adding 50 mM Na+ to the artificial seawater caused an amiloride-sensitive pHi increase under both normo- and hypertonic conditions. The computed Na-H exchange flux (JNa-H) increased with increasing [Cl-]i under both normo- and hypertonic conditions, with similar apparent Km values ( approximately 120 mM). However, the maximal JNa-H increased by nearly 90% under hypertonic conditions. Thus, activation of Na-H exchange at low pHi requires Cl- under both normo- and hypertonic conditions, but at any given [Cl-]i, JNa-H is greater under hyper- than normotonic conditions. We conclude that an increase in [Cl-]i is not the primary shrinkage signal, but may act as an auxiliary shrinkage signal. To determine whether the Cl--dependent step is after the G-protein-dependent step, we predialyzed BMFs to a Cl--free state, and then attempted to stimulate Na-H exchange by activating a G protein. We found that, even in the absence of Cl-, dialyzing with GTPgammaS or AlF3, or injecting cholera toxin, stimulates Na-H exchange. Because Na-H exchange activity was absent in control Cl--depleted fibers, the Cl--dependent step is at or before the G protein in the shrinkage signal-transduction pathway. The stimulation by AlF3 indicates that the G protein is a heterotrimeric G protein.
Hogan
E M
EM
Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06520, USA.
Davis
B A
BA
Boron
W F
WF
D017607:Aluminum Compounds / Q000494:pharmacology; D000818:Animals; D002712:Chlorides / Q000378:metabolism*; D002772:Cholera Toxin / Q000494:pharmacology; D005459:Fluorides / Q000494:pharmacology; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D006982:Hypertonic Solutions / Q000494:pharmacology*; D007425:Intracellular Membranes / Q000378:metabolism*; D018485:Muscle Fibers, Skeletal / Q000166:cytology / Q000378:metabolism*; D012016:Reference Values; D017923:Sodium-Hydrogen Exchangers / Q000187:drug effects / Q000378:metabolism*; D001468:Thoracica / Q000378:metabolism*
D017607:Aluminum Compounds; D002712:Chlorides; D006982:Hypertonic Solutions; D017923:Sodium-Hydrogen Exchangers; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D002772:Cholera Toxin; D005459:Fluorides; C032311:aluminum fluoride