Abstract
The ion transport properties and the adrenergic control of secretion were studied in the isolated opercular epithelium of sea water-adapted Fundulus grandis. The opercular epithelium is an important model epithelium to use in studying the functions of chloride cells in the gills of sea water teleosts since: (1) it is identical in ultrastructure and surface morphology to the salt-secretory epithelium of the gill filament; (2) it contains a much higher density of chloride cells--the proposed active secretory cell type; and (3) it can be isolated as a flat sheet epithelium and mounted in an Ussing-type chamber where stringent thermodynamic conditions can be maintained.When bathed bilaterally with modified Forster's Ringer and gassed with 95% O(,2)/5% CO(,2), the isolated opercular epithelium of sea water-adapted F. grandis generates a mean transepithelial potential difference (P.D.) of 8.2 (+OR-) 0.4 mV (serosa positive), a mean short-circuit current (I(,sc)) of 52.8 (+OR-) 3.6 uA('(.))cm('-2) and a mean transepithelial resistance (R) of 194.1 (+OR-) 8.6 ohm(.)cm('2) (mean (+OR-) s.e.m., n = 111). The electrical parameters are the result of the net, active secretion of chloride, sodium is passively distributed across the epithelium.The I(,sc), indicative of the rate of active chloride secretion, is decreased 94.8 and 82.7% by the serosally administered transport inhibitors, ouabain and furosemide, respectively. On the other hand, SITS, acetazolamide, and amiloride have little or no effects on the I(,sc).Results of voltage-clamp studies suggest that sodium movements are entirely passive in nature and proceed through the low resistance paracellular shunt pathway. The ratio of the permeability of chloride to that of sodium through this pathway is 0.21. This ratio is significantly less than the ratio of 1.52 predicted for free solution at 25(DEGREES)C and suggests that the paracellular pathway is cation selective. In addition, the conductance of the paracellular pathway accounts for approximately 83% of the total tissue conductance, thus classifying the opercular epithelium as a "leaky" epithelium.The transport of chloride across the apical membrane of chloride cells is not dependent upon the presence of HCO(,3)('-) in the mucosal medium, but is stimulated by mucosal alkaline pH's. These results, taken in conjunction with the lack of a significant acetazolamide effect, suggest that the chloride exit step is not via a Cl('-)/HCO(,3)('-) exchange mechanism but may be diffusional in nature.All of these observations are consistent with and provide more direct evidence favoring the Silva et al (1977a) model of ion transport by chloride cells in sea water teleosts.Active chloride secretion by the opercular epithelium is regulated both by (alpha)- and (beta)-adrenergic receptor stimulation. Whereas (beta)-receptor activation effects are stimulatory, the activation of (alpha)-receptors leads to a complete inhibition of active chloride secretion. While the intracellular mediator of (beta)-receptor activation in C-AMP, the intracellular mediator of (alpha)-receptor activation is not known with certainty, but evidence suggests that it may be C-GMP.Ca('2+) does not appear to be involved in (alpha)-receptor modulation but may act at a site distal to that of C-AMP. Thus, Ca('2+) appears to be a "tertiary" intracellular messenger in controlling Cl('-) secretion by the opercular epithelium.