Recent evidence, principally from electrical measurements, indicates that transmitter substances (e.g. acetylcholine) act at synapses by altering the permeability of the post-synaptic membrane to one or more inorganic ions. Such a mechanism was first proposed by Fatt & Katz (1951) to account for the depolarizing action of acetylcholine (ACh) at the end-plate region of skeletal muscle. The aim of the present work was to obtain more information about the nature of the change in permeability underlying the action of a stable analogue of ACh, carbachol, on mammalian smooth muscle. For this purpose, the movement of labelled inorganic ions was examined in preparations bathed in solutions containing sufficient potassium to depolarize the muscle fibres; the effects of carbachol on permeability could then be studied without the secondary changes in ion flux associated with the depolarization, and consequent increase in spike activity, which otherwise result from the action of the drug. A preliminary account of part of these results has been given to the Physiological Society (Durbin & Jenkinson, 1959). METHODSThe taenia coli of the guinea-pig was used in these experiments, which were made at room temperature (20-23o C). The animal was killed by a blow on the head and the required amount of the taeniae excised immediately. It was found difficult to separate the muscle from the large intestine without removing at the same time some of the underlying tissue. Most of this was subsequently detached with fine scissors under a dissecting microscope. During dissection, the strips of taenia were bathed in a modified Krebs's solution which had the following composition (mM): NaCl 133, KCI 5-6, NaHCO3 16, CaCl2 2B5, MgCI1 1-15, glucose 6, Na phosphates 0-5, and was bubbled with a gas mixture containing 95 % 02 and 5 % CO2. After preparation, the strips were stretched to approximately their in situ length on frames of stainless-steel wire. They were then placed in a solution in which all but 5 mms of the NaCl had been replaced by K2S04. It was found in preliminary experiments that immersion in potassium-rich fluids containing the full osmotic equivalent of the replaced NaCl caused the muscles to lose weight rather rapidly. For this reason, less K2SO4 was included in the potassium-rich solution finally employed, which had the following composition (mM); K,SO2 76, KHCO3 16, CaCl, 7-5, Na2SO4 2-5, MgCls 1-15, glucose 6, Na * Established Investigator of the American Heart Association. Present address: Biophysical Laboratory, Harvard Medical School, Boston 15, Mass., U.S.A.

Direct measurements have been made of the net volume flow through cellulose membranes, due to a difference in concentration of solute across the membrane. The aqueous solutions used included solutes ranging in size from deuterated water to bovine serum albumin. For the semipermeable membrane (impermeable to the solute) the volume flow produced by the osmotic gradient is equal to the flow produced by the hydrostatic pressure RT AC, as given by the van't Hoff relationship. In the case in which the membrane is permeable to the solute, the net volume flow is reduced, as predicted by the theory of Staverman, based on the thermodynamics of the steady state. A means of establishing the amount of this reduction is given, depending on the size of the solute molecule and the effective pore radius of the membrane. With the help of these results, a hypothetical biological membrane moving water by osmotic and hydrostatic pressure gradients is discussed.The processes of secretion and absorption in the body have often been assumed to require a movement of water in response to an osmotic gradient. By this hypothesis, a gradient in water activity is produced by the absorption or secretion of a solute. Usually this solute is a small ion or molecule, to which the membrane may be permeable. The influence of a given concentration difference of the solute on the osmotic movement of water depends on the extent of such permeability, or "leakiness." Staverman (1), in a theoretical treatment of this problem using the methods of irreversible thermodynamics, has introduced a correction factor or into the van't Hoff expression for the osmotic pressure due to the solute. In the present work, the Staverman factor or has been determined for a number of different solutes, in conjunction with three cellulose membranes which have been physically characterized. A systematic means of predicting or from the properties of the membrane is given, and the use of or in constructing models of the transporting membrane Dr.

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