Abstract
The isolated, perfused heads (IPHP) of Opsanus beta and Myoxocephalus octodecimspinosus were used in the study of gill perfusion regulation and transbranchial salt transport. The IPHP allows control of both serosal and mucosal sides of the branchial epithelium, which is not possible in "whole animal" studies. IPHPs of both species exhibit a long viability and extended stability of gill resistance not found in other fish.The partitioning of flow through the branchial vasculature is via alpha adrenergic control, while the decrease in gill resistance elicited by epinephrine is governed by beta receptors. The combined effect of these two mechanisms during epinephrine treatment is a shunting of perfusate away from efferent venous pathways and a net drop in branchial resistance to perfusion.The transbranchial transfer of ammonia from the perfusate to the seawater may depend on both serosal to mucsal diffusion of NH(,4)('+) and the transport of NH(,4)('+) by basolateral Na('+)-K('+) ATPase.Na and Cl efflux from the IPHPs appears to occur through two different rate limiting pathways since Na but not Cl efflux is affected by perfusion rate modifications. NaCl efflux is not altered by gill irrigation rate changes. The transepithelial potential in the IPHP of M. octodecimspinosus is similar to that recorded in vivo, while the gills, both in vitro and in vivo, are more permeable to Na than to Cl. A reduced Cl permeability in the IPHP may be responsible for an observed Cl efflux which is lower than in vivo efflux rates. Perfusate ouabain addition inhibits the efflux of Na from the IPHP of O. beta, while the addition of K to the perfusate stimulates Cl efflux. Furosemide and K addition to the perfusate of the IPHP of M. octodecimspinosus have no effect on the rate of Cl efflux from the preparation. An increase in Cl efflux is observed in this species after cAMP addition. These results are discussed in relation to current models of transbranchial NaCl movement.