Transepithelial transport of zinc and L-histidine across perfused intestine of American lobster, Homarus americanus

The intestine of the American lobster, Homarus americanus, was isolated and perfused in vitro with a physiological saline, based on the ion composition of the blood, to characterize the mechanisms responsible for transmural transport of zinc and how the amino acid, L-histidine, affects the net movement of the metal across the tissue. Previous studies with this preparation, focusing on the characteristics of unidirectional mucosa to serosa (M to S) fluxes of Zn-65(2+) and H-3-L-histidine, indicated the presence of a brush border co-transport process responsible for simultaneously transferring the metal and amino acid across this tissue as an apparent bis-complex (Zn-[His](2)) using a PEPT-1-like dipeptide carrier mechanism. In addition, both zinc and L-histidine were also transferred toward the blood by separate transporters that were independent of the other substrate. The focus of the present study was to characterize the serosa to mucosa (S to M) flux of Zn-65(2+) under a variety of conditions, and use these values in conjunction with those from the previous study, to assess the direction and magnitude of net metal movement across the tissue. Transmural S to M transport of Zn-65(2+) was markedly reduced with the addition of the serosal inhibitors ouabain (32%), excess K+ (25%), excess Ca2+ (30%), Cu2+ (38%), nifedipine (21%), and vanadate (53%). In contrast, this flux was markedly stimulated with the serosal addition of ATP (24%) and excess Na+ (28%). These results suggest that S to M fluxes of zinc occurred by the combination of the basolateral Na/Ca exchanger (NCX), where zinc replaced calcium, and a basolateral nifedipine-sensitive calcium channel. Transmural M to S Zn-65(2+) fluxes (5-100 mu M) were threefold greater than S to M metal transport, and the addition of luminal L-histidine doubled the net M to S zinc flux over its rate in the absence of the amino acid. The results of this paper and those in its predecessor indicate that zinc transport by the lobster intestine is absorptive and significantly enhanced by luminal amino acids.