Diffusion of drugs through stationary water layers as the rate limiting process in their action at membrane receptors

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Abstract
1 . Preparations bathed in a well stirred solution have been considered as heterogeneous systems in which the solid phase is enveloped by a thin layer of stationary liquid. Any substance applied into the bulk solution must pass through this layer by diffusion before reaching the receptors. 2 . The rate of diffusion through the stationary layer can govern the time course of the cellular responses to applied drugs provided that (i) all receptors involved in the response are situated at an equal distance from the solution and (ii) interaction with the receptor and consequent cellular events are very rapid. 3 . These conditions have been verified for two responses: the contraction of guinea-pig ileum by acetylcholine (ACh), carbamylcholine (CCh), histamine and KCl, and the depolarization of the rat isolated sympathetic ganglion by ACh in the presence of eserine. A method of analysis has been applied which allows a complete dose-response curve to be obtained from only two responses. 4 . Diffusion half-times measured for pieces of ileum were 4·13 ± 0·13 s (s.e. of mean) for ACh, 3·60 ± 0·05 s for CCh and 1·01 ± 0·05 s for KCl. The equivalent thickness of the stationary layer calculated from these values was respectively 93 μm, 87 μm and 70 μm. The average diffusion half-time for ACh in sympathetic ganglia was 14·19 ± 1·05 s. This gives an equivalent thickness of 173 μm. 5 . Diffusion half-times were increased by increasing the viscosity of the bathing solution without changing the concentration response relationship. 6 . The time course of contractions of guinea-pig ileum are no longer diffusion limited in the presence of a competitive antagonist or when the temperature is lowered from 35° to 25° C.

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