Ion and Water Balance in the Ixodid Tick Dermacentor Andersoni

Abstract

ABSTRACT The salivary gland of the ixodid tick, Dermacentor andersoni, can be induced to secrete fluid for at least 6 h when bathed in an artificial medium in vitro. Fluid secretion appears to be a consequence of active Cl secretion since (a) it is inhibited by 95 % when nitrate and by 100% when acetate replaces Cl in the bathing medium ; however, bromide can support secretion as well as Cl, (b) the rates of fluid and Cl secretion are linearly related to the concentration of Cl in the medium; and (c) the S/H ratio for Cl is greater than unity at all concentrations despite a transacinar P.D. of 35 mV (lumen negative). Although (in the presence of Na) a low concentration of K in the bathing medium stimulates the rate of fluid secretion fivefold, higher concentrations of K inhibit fluid secretion. The latter is largely due to a direct effect of K ion and not simply to increased osmotic pressure or reduced Na concentration. Fluid secretion is completely inhibited by 10−6 M ouabain. On the basis of these observations we propose that fluid secretion may be dependent on a Na-K activated ‘pump ATPase ‘, which is somehow involved in cation secretion. The S/H ratios of Na and K are greater than unity at all medium concentrations. The saliva secreted in vitro is slightly hypo-osmotic to the bathing medium over a wide range of medium concentration (300–920 mOsm/1). We postulate that the primary saliva is iso- or hyper-osmotic to the bathing medium; the final elaborated saliva is probably rendered hypo-osmotic by a process of solute reabsorption somewhere between the acini and the orifice of the main salivary duct.