PubMed · 994497
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C S Avila, W M Gooch. 1976. Ionized calcium.. https://pubmed.ncbi.nlm.nih.gov/994497/
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The purpose of this work was to characterize the gap junctions between cultured glomus cells of the rat carotid body and to assess the effects of acidity and accompanying changes in [Ca(2+)](i) on electric coupling. Dual voltage clamping of coupled glomus cells showed a mean macrojunctional conductance (G(j)) of 1.16 nS+/-0.6 (S.E.), range 0.15-4.86 nS. At normal pH(o) (7.43), a steady transjunctional voltage (DeltaV(j)=100.1+/-10.9 mV) showed multiple junction channel activity with a mean microconductance (g(j)) of 93.98+/-0.6 pS, range 0.3-324.5 pS. Single-channel conductances, calculated as variance/mean g(j), gave a mean value of 16.7+/-0.2 pS, range 5.13-39.38 pS. Manual measurements of single-channel activity showed a mean g(j) of 22.03+/-0.2 pS, range 1.3-160 pS. Computer analysis of the noise spectral density distribution gave a channel mean open time of 12.7+/-1.5 ms, range 6.37-23.42 ms. The number of junction channels, estimated in each experiment from G(j)/single-channel g(j), showed a range of 7 to 258 channels (mean, 107.2). Optical measurements of [Ca(2+)](i) gave a mean value of 80.2+/-4.27 nM at pH(o) of 7.43. Acidification of the medium with lactic acid (1 mM, pH 6.3) induced: 1) Variable changes in G(j) (decreases and increases); 2) A significant decrease in mean g(j) (to 80.36+/-0.34 pS) and in single-channel conductance (g(j)=12.8+/-0.2 pS in computer analyses and 17.23+/-0.2 pS when measured by hand); 3) Variable changes in open times, resulting in a similar mean (12.8+/-1.5 ms) and 4) No change in the number of junction channels. When pH(o) was lowered to 6.3 [Ca(2+)](i) did not change significantly (there were increases and decreases). However, when pH(o) was lowered to 4.4, [Ca(2+)](i) increased significantly to 157.1+/-8.1 nM. It is concluded that saline acidification to pH 6.3 depresses the conductance of junction channels and this effect may be either a direct effect on channel proteins or synergistically enhanced by increases in [Ca(2+)](i). However, there are no studies correlating changes of [Ca(2+)](i) and intercellular coupling in glomus cells. Stronger acidification (pH(o) 4.4), producing much larger changes in [Ca(2+)](i), may enhance this synergism. But, again, there are no studies correlating these effects.
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The sodium bicarbonate cotransporter (NBC1) is essential for bicarbonate transport across plasma membranes in epithelial and nonepithelial cells. The direction of the NaHCO3 movement in secretory epithelia is opposite to that in reabsorptive epithelia. In secretory epithelia (such as pancreatic duct cells) NBC is responsible for the transport of bicarbonate from blood to the cell for eventual secretion at the apical membrane. In reabsorptive epithelia (such as kidney proximal tubule cells) NBC is responsible for the reabsorption of bicarbonate from cell to the blood. In nonepithelial cells this transporter is mainly involved with cell pH regulation. Recent molecular cloning experiments have identified the existence of four NBC isoforms (NBC1, 2, 3 and 4) and two NBC-related proteins AE4 and NCBE (Anion Exchanger 4 and Na-dependent Chloride-Bicarbonate Exchanger). All but AE4 are presumed to mediate the cotransport of Na+ and HCO(3-) under normal conditions and may be functionally altered in certain pathologic states. NBC1 shows a limited tissue expression pattern, is electrogenic and plays an important role in bicarbonate reabsorption in kidney proximal tubule. In addition to the kidney, NBC1 is expressed in pancreatic duct cells, is activated by cystic fibrosis transmembrane conductance regulator (CFTR) and plays an important role in HCO3- secretion. NBC2 and NBC3 have a wider tissue distribution than NBC1, are electroneutral, and are involved with cell pH regulation. The characterization of NBC4 is incomplete. The NBC-related protein called NCBE mediates Na-dependent, Cl-/Bicarbonate Exchange. The purpose of this review is to summarize recent advances on the cloning of NBC isoforms and related proteins and their role and regulation in physiologic and pathologic states.