Differential Sensitivity to Intracellular pH Among High- and Low-Threshold Ca2+ Currents in Isolated Rat CA1 Neurons

• Published in: Journal of neurophysiology 1997-02, Vol.77 (2), p.639-653
• Main Authors: Tombaugh, Geoffrey C, Somjen, George G
• Format: Article
• Language: English
• Subjects: Animals; Calcium Channels - physiology; Hippocampus - physiology; Hydrogen-Ion Concentration; Male; Patch-Clamp Techniques; Pyramidal Cells - physiology; Rats; Rats, Sprague-Dawley
• ISSN: 0022-3077; 1522-1598
• DOI: 10.1152/jn.1997.77.2.639

Geoffrey C. Tombaugh and George G. Somjen Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710 Tombaugh, Geoffrey C. and George G. Somjen. Differential sensitivity to intracellular pH among high- and low-threshold Ca 2+ currents in isolated rat CA1 neurons. J. Neurophysiol. 77: 639-653, 1997. The effects of intracellular pH (pH i ) on high-threshold (HVA) and low-threshold (LVA) calcium currents were examined in acutely dissociated rat hippocampal CA1 neurons with the use of the whole cell patch-clamp technique (21-23°C). Internal pH was manipulated by external exposure to the weak base NH 4 Cl or in some cases to the weak acid Na-acetate (20 mM) at constant extracellular pH (7.4). Confocal fluorescence measurements using the pH-sensitive dye SNARF-1 in both dialyzed and intact cells confirmed that NH 4 Cl caused a reversible alkaline shift. However, the external TEA-Cl concentration used during I Ca recording was sufficient to abolish cellular acidification upon NH 4 Cl wash out. With 10 mM N -2-hydroxyethylpiperazine- N -2-ethanesulfonic acid (HEPES) in the pipette, NH 4 Cl exposure reversibly enhanced HVA currents by 29%, whereas exposure to Na-acetate markedly and reversibly depressed HVA Ca currents by 62%. The degree to which NH 4 Cl enhanced HVA currents was inversely related to the internal HEPES concentration but was unaffected when internal ethylene glycol-bis( -aminoethyl ether)- N,N,N ,N -tetraacetic acid (EGTA) was replaced by equimolar bis-( o -aminophenoxy)- N,N,N ,N -tetraacetic acid (BAPTA). When depolarizing test pulses were applied shortly after break-in ( V h  =  100 mV), NH 4 Cl caused a proportionally greater increase in the sustained current relative to the peak. The dihydropyridine Ca channel antagonist nifedipine (5 µM) blocked nearly all of this sustained current. A slowly inactivating nifedipine-sensitive (L-type) HVA current could be evoked from a depolarized holding potential of 50 mV; NH 4 Cl enhanced this current by 40 ± 3% (mean ± SE) and reversibly shifted the tail-current activation curve by +6-8 mV. L-type currents exhibited more rapid rundown than N-type currents; HVA currents remaining after prolonged cell dialysis, or in the presence of nifedipine, inactivated rapidly and were depressed by -conotoxin (GVIA). NH 4 Cl enhanced these N-type currents by 76 ± 9%. LVA Ca currents were observed in 32% of the cells and exhibited little if any rundown. These amiloride-sensitive currents activ