The effect of natural and synthetic vasopressins and other substances on active transport in ciliary epithelium of the rabbit

Abstract

The present experiments were undertaken in order to see whether natural and synthetic vasopressins alter the rate of active sodium transport in vitro under conditions independent of blood supply using a preparation of isolated ciliary body from the rabbit and measuring changes of trans-epithelial short-circuit current the chief component of which (at least in this species) is the “active” sodium flux ( Cole, 1962). Preliminary studies were also made on the effects of catecholamines, pilocarpine, acetylcholine and prostaglandins E 1 and F 2 α . The iris/ciliary body preparation from albino rabbits was mounted between 2 chambers containing modified Krebs' medium and maintained at 37°C. Trans-epithelial potential was recorded under open-circuit conditions throughout the experiment and short-circuit current was measured at intervals of 5 to 15 min. After an initial control period the drug under investigation was added to the medium on the cameral side of the tissue and the measurements continued for a further 15 to 30 min before repeating the procedure at the next higher dose-level. Three or four different concentrations were used with each preparation and the responses, expressed as percentage change of short-circuit current from mean control value were used to derive dose/response curves. The substances used were natural vasopressin, lysine 8-vasopressin, phenylalanine 2-lysine 8-vasopressin and ornithine 8-vasopressin, l-noradrenaline bitartarate, isoprenaline hydrochloride, pilocarpine, nitrate, prostaglandins E 1 and F 2 α (Upjohn) and 3′5′ adenosine monophosphate. Natural vasopressin and Lys 8-vasopressin both caused significant increases of short-circuit current at concentrations less than 10 μU/ml. The 2 other substances were less active, Orn 1-vasopressin being effective only at 10 5 μU/ml. On the basis of their effectiveness in stimulating active transport in the present experiments the 4 substances may be placed in the following order: “Pitressin” ≙ Lys 8-vasopressin > Phe 2Lys 8-vasopressin > Orn 8-vasopressin which would accord with their ranking in order of antidiuretic activity. Neither catecholamines pilocarpine or acetylcholine showed direct inhibitory action on active transport and it is suggested that any ocular hypotensive action is due to haemodynamic changes or to alterations of outflow facility. The prostaglandins stimulated short-circuit current but cyclic-AMP had little consistent action. If, as these findings indicate, active transport across the ciliary epithelium can be stimulated by vasopressin concentrations within the normal physiological range of 1 to 10 μU/ml, it seems reasonable to infer that in intact animals, the level of endogenous vasopressin production may be significant in regulating the rate of aqueous humour formation.