The role of K+ and Cl- conductances in chloride secretion by the opercular epithelium

Micromolar copper modifies electrical properties and spontaneous discharges of nodose ganglion neurons in vitro

Copper plays a key role in aerobic cell physiology mainly related to mitochondrial metabolism. This element is also present at higher than basal levels in some central nuclei and indeed, current evidence support copper's role as a neuromodulator in the central nervous system. More recent data indicate that copper may also affect peripheral neuronal activity, but so far, there are not detailed descriptions of what peripheral neuronal characteristics are targeted by copper. Here, we studied the effect of physiological concentration of CuCl2 (μM range) on the activity of peripheral neurons using a preparation of nodose ganglion in vitro. By mean of conventional intracellular recordings passive and active electrical membrane properties were studied. Extracellular copper modified (in a redox-independent manner) the resting membrane potential and the input resistance of the nodose ganglion neurons, increasing the excitability in most of the tested neurons. These results suggest that Cu(2+) modulates the activity of nodose ganglion neurons and support nodose ganglion in vitro preparation as a simple model to study the subcellular mechanisms involved in the Cu(2+) effects on neuron electrical properties.

Epithelial mitochondria-rich cells and associated innervation in adult and developing zebrafish

Studies of ion regulation by mitochondria-rich cells (MRCs) of transport epithelia in fish have revealed many processes by which ion homeostasis is achieved. However, the control of these mechanisms and, particularly, the extent of nervous system involvement are not completely understood. We characterized the potential innervation of MRCs in various gill and extrabranchial tissues involved in ion transport in the model vertebrate the zebrafish. Confocal and conventional microscopy of whole-mount preparations were combined with immunofluorescence techniques to label MRCs with antibodies against a subunit of the enzyme Na(+)/K(+)-ATPase and nerve fibers with a zebrafish neuronal marker, zn-12. MRCs of the gill filaments were identified by their morphology and migration out to the lamellae in response to ion-poor water acclimation. Gill MRCs were intimately associated with nerve fibers originating from outside the filaments. MRCs of the opercular epithelium resembled those of the gill and were also located adjacent to nerve fibers. Mitochondria-rich "pseudobranch cells" were identified in the pseudobranch by immunofluorescence and labeling of dissociated cells with the mitochondrial marker DASPEI. Pseudobranch MRCs resembled gill MRCs and received innervation from a dense network of nerve fibers. In larvae, MRCs were distributed across the surface of the skin. These cells were situated among a dense network of varicose nerve fibers, and some MRCs of the skin displayed extensive cytoplasmic processes. Evidence is presented suggestive of widespread association of MRCs with the nervous system in transport epithelia and the neural control of MRC-mediated ion regulation in teleost fish.

The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste

The fish gill is a multipurpose organ that, in addition to providing for aquatic gas exchange, plays dominant roles in osmotic and ionic regulation, acid-base regulation, and excretion of nitrogenous wastes. Thus, despite the fact that all fish groups have functional kidneys, the gill epithelium is the site of many processes that are mediated by renal epithelia in terrestrial vertebrates. Indeed, many of the pathways that mediate these processes in mammalian renal epithelial are expressed in the gill, and many of the extrinsic and intrinsic modulators of these processes are also found in fish endocrine tissues and the gill itself. The basic patterns of gill physiology were outlined over a half century ago, but modern immunological and molecular techniques are bringing new insights into this complicated system. Nevertheless, substantial questions about the evolution of these mechanisms and control remain.

Rapid regulation of NaCl secretion by estuarine teleost fish: coping strategies for short-duration freshwater exposures

This review summarizes the mechanism of Cl(-) active secretion and its regulation in estuarine teleost fish. Small estuarine fish such as the killifish, Fundulus heteroclitus, forage in shallow water following advancing tides and are exposed regularly to very dilute microenvironments. Using the killifish opercular epithelium and related teleost membranes containing mitochondria-rich cells, the regulation includes a reduction of active Cl(-) secretion and passive diffusive ion loss in a three-stage process spanning approximately 30 min. There is a combination of sympathetic neural reflex mediated by alpha(2)-adrenoceptors operating via intracellular inositol tris phosphate and intracellular Ca(2+) and a cellular hypotonic shock response, followed by covering over of ion-secreting cells by pavement cells. This effectively minimizes salt loss in dilute media. The upregulation of salt secretion on return to full strength seawater may be via hormones (arginine vasotocin and urotensin I) and neurotransmitter (vasoactive intestinal polypeptide) in combination with hypertonic shock. A hypothetical model includes involvement of protein kinase A and C and protein phosphatases 1 and 2A in regulation of the NKCC1 cotransporter on the basolateral side and protein kinase A regulation of the CFTR-like apical anion channel.

Transport mechanisms of seawater teleost chloride cells: an inclusive model of a multifunctional cell

This review assembles recent information on seawater-type chloride cells of marine teleost fish and evaluates the secretion of Na+, Cl-, K+, H+ and NH4+ and the absorption of Ca2+. The evidence for the distribution (apical vs basolateral) and the abundance of the various ion pumps, cotransporters, channels and exchangers is assessed and an inclusive model is constructed. Relationships among the transport systems are presented to suggest that many, if not all, of these systems may be operating simultaneously in individual, multifunctional chloride cells.

Cyclic AMP stimulation of Cl- secretion by the opercular epithelium: the basolateral chloride uptake mechanism

The isolated, short-circuited opercular epithelium of Fundulus heteroclitus, secretes Cl- by a mechanism dependent on the presence of serosal Na+ and inhibited by bumetanide and furosemide. Under serosal Na+-free conditions the active Cl- secretion is abolished. However, subsequent elevations of intracellular cyclic AMP (cAMP) levels with isoproterenol or forskolin stimulated Cl- secretion markedly. This stimulation was unaffected by SITS, DIDS, methazolamide, and HCO-3-free solutions, but was blocked by furosemide and bumetanide. Determinations of relative intracellular 36Cl- levels showed a Na+ dependence of intracellular 36Cl- in epithelia not stimulated by isoproterenol and a Na+ independence of intracellular 36Cl- in isoproterenol stimulated epithelia. In both conditions, the intracellular 36Cl- was bumetanide sensitive. The results indicate that cAMP stimulation of Cl- secretion can occur by a Na+-independent, loop diuretic-inhibitable mechanism, which may be operative even in the presence of Na+. Whether this is a separate Cl- uptake mechanism or a cAMP-induced alteration in the normal Na+-dependent mechanism could not be determined. In either instance, an alternative to the Na+ gradient as a source of energy for Cl- uptake into the cell across the basolateral membrane is required.

Cyclic AMP stimulation of Cl- secretion by the opercular epithelium: the apical membrane chloride conductance

Chloride secretion (Isc) by the opercular epithelium of the teleost, Fundulus heteroclitus, is stimulated by elevations in intracellular cyclic AMP (cAMP) elicited by beta-adrenergic agonists, such as isoproterenol, and is accompanied by a small but significant increase in the transepithelial conductance (Gt). Cupric ions (Cu2+) have been shown to block the apical membrane Cl- channels in this epithelium, leading to a reduction in both the Isc and Gt (Degnan, '85). In the present studies, the effects of Cu2+ on cAMP-elevated and cAMP-depleted epithelia were observed to define the actions of cAMP in this stimulatory process. At a concentration of 5 X 10(-4) M in the mucosal solution, Cu2+ inhibited the Isc 79.8% and reduced the Gt 39.2%. Isoproterenol produced an attenuated stimulation of the Isc in these tissues compared to untreated controls, but had no effect on the Gt. In tissues bathed bilaterally with Cl- -free Ringer, the Isc was virtually abolished and the Gt was reduced 37.0%; neither Cu2+ nor isoproterenol had any effects on the Isc or Gt under this condition. Simultaneous 2 2Na and 3 6 Cl unidirectional flux determinations indicated that the only effects of both isoproterenol and Cu2+ were on the active Cl- secretory flux. An inhibitor of adenylate cyclase, 2',5' dideoxyadenosine (DDA), reduced the Isc and Gt 39.8% and 20.8% respectively. This inhibitor had no additional effects in Cu2+ -treated tissues and the action of Cu2+ on the Gt was reduced in DDA-treated tissues.(ABSTRACT TRUNCATED AT 250 WORDS)