Cutaneous Ion Transport in the Freshwater Teleost Synbranchus marmoratus

• Published in: Physiological zoology 1986-07, Vol.59 (4), p.406-418
• Main Authors: Stiffler, Daniel F., Graham, Jeffrey B., Dickson, Kathryn A., Stöckmann, Wieland
• Format: Article
• Language: English
• Subjects: Air; Biological and medical sciences; Breathing; Dendritic cells; Excretion; Freshwater; Fundamental and applied biological sciences. Psychology; Gills; Intestines; Ion transport; Skin; Sodium; Synbranchus marmoratus; Urine; Vertebrates: skin, associated glands, phaneres, light organs, various exocrine glands (salt gland, uropygial gland...), adipose tissue, connective tissue
• ISSN: 0031-935X; 1937-4267
• DOI: 10.1086/physzool.59.4.30158594

The gills of Synbranchus marmoratus become isolated from water for long periods of time while this fish breaths air or resides in moist burrows. In order to evaluate how interruptions in gill/water contact affect ionic regulation, we have undertaken urine collections and measurements of net and unidirectional sodium fluxes on intact, conscious fish. Comparative ionic flux measurements were made on the lungfishes Lepidosiren paradoxa and Protopterus spp. All three species demonstrated Na⁺ influx rates that are low for fish and freshwater vertebrates in general. Synbranchus marmoratus has an affinity ($K_{m}$) of 0.45 mM and a capacity ($J_{max}$) of 41.1 μEq/kg h⁻±, suggesting a relatively low-affinity, low-capacity ion transport system. The transepithelial potential difference measured between the bath and extracellular fluid was -7 mV (inside reference); thus the fluxes could not be accounted for by passive transport mechanisms (Ussing criterion). Synbranchus marmoratus has both a low urine flow (1.8 ml/kg h⁻±) and a low urinary Na⁺ excretion rate (17.9 μEq/kg h⁻±), which, when combined with the extrarenal Na⁺ loss (3.9 μEq/kg h⁻±), indicate that it has a very low rate of turnover of Na⁺. Interruptions of branchial/water contact during air breathing do not affect influx or efflux of sodium. Experiments that partitioned Na⁺ fluxes in the head (gill and skin) from those in the body (skin and urinary) showed that 75% of the influx occurred across the body surface. We therefore conclude that S. marmoratus utilizes its skin as an ion-transporting organ. Histologic studies of this fish reveal structural characteristics common to other ion-transporting epithelia, such as junctional complexes, apical microridges, and mitochondria-rich cells.