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Role of the salivary gland in ion and water regulation during feeding in the female tick, dermacentor andersoni Kaufman, William Reuben

Abstract

Of the total water lost during the feeding period by Dermacentor andersoni females, 75% was excreted in the saliva, less than 5% was evaporated from the integument, and the remainder was lost in the feces. Ninety-five percent of the total excreted sodium and 18% of the total excreted potassium was lost in the saliva. The complementary percentages of the latter ions were excreted in the feces. Saliva was shown to be produced by a secretory rather than a filtration mechanism. Since salivary glands set up in vitro secreted fluid when offered adrenalin, noradrenalin or dopamine, but not when offered natural hemolymph from salivating ticks, I proposed that secretion is normally under neural rather than endocrine control. Moreover, since In-vitro glands were insensitive to pilocarpine and relatively insensitive to serotonin, this suggested that the natural transmitter substance may be a catecholamine. The presence of nerves in close association with salivary gland tissue was confirmed histologically and ultrastrueturally. Fluid transport ceased when chloride in the external medium was replaced with acetate, and was inhibited by 95% when replaced with nitrate. The rate of fluid secretion was proportional to the chloride concentration In the external medium. The electropotential difference recorded across the salivary acini by means of glass microelectrodes was about 30 mV lumen negative, and the saliva to hemolymph ratio for chloride in vitro was about 1.6. These data indicated that chloride is actively transported by the salivary epithelium. The rate of fluid transport was Influenced by the Na:K ratio in the medium. The rate of fluid secretion was maximal in medium containing 210 meq/llter Na and 10 meq/liter K. Replacing the 10 meq/liter K with Na inhibited the secretory rate by 80$. However the progressive increasing of the K level beyond 10 meq/liter at the expense of Na also inhibited the rate of fluid secretion, leading to complete cessation when both cations were in equimolar concentration. Fluid transport also ceased when glands were bathed in normal Ringer solution containing 10⁻⁶ M ouabain. The latter results suggested that the energy required for fluid secretion comes from a Na,K-activated ‘pump ATPase'. The salivary gland is composed of three distinct types of acinus (types I, II, III). The type I acinus is composed of two cell types and the types II and III acini five cell types each. The ultrastructural features of the cell named here the "water cell” in the type III acinus are very similar to those described for other fluid-transporting epithelia. The maze of intracellular channels associated closely with numerous mitochondria suggested that fluid transport could be explained by the standing-gradient hypothesis of Diamond and Bossert (I967, 1968). But the latter model can only explain the transport of an iso-osmotic or a hyperosmotic fluid. Since the fluid secreted by in-vltro salivary glands was hypo-osmotic to the bathing medium under all conditions tested, the primary fluid secreted into the acinus must be subsequently rendered hypo-osmotic by means of solute reabsorption. The site of reabsorption was not localized, but reabsorption was suspected to occur in the duct system, either by the duct cells themselves as in mammals, or conceivably by the fibrillar cells of the type I acinus.

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