The effect of carbachol on the permeability of depolarized smooth muscle to inorganic ions

Abstract

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 They were then placed in a solution in which all but 5 mms of the NaCl had been replaced by K2S04.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.

Movements of radioactive potassium in isolated rat ganglia

“…In this context, the magnitude and time course of the carbachol-or nicotine-induced changes in 42K content accord reasonably well with previously observed values determined by flame photometry (Brown & Scholfield, 1974a Brown, personal communication). This contrasts with the situation in K-depolarized smooth muscle or denervated skeletal muscle (Durbin & Jenkinson, 1961;Jenkinson & Nicholls, 1961). Perhaps in Kdepolarized ganglia, 42K efflux is limited by some factor other than neuronal permeability; alternatively, the permeability changes produced by nicotinic agonists may be reduced during depolarization (Adams, 1975;Lester, Changeux & Sheridan, 1975).…”

Effects of Noradrenaline and Isoprenaline on the Electrical and Mechanical Activities of Guinea Pig Depolarized Taenia Coli

“…The consequent block of the spike generation then results in the relaxation of the muscle, in the manner proposed by JENKINSON and MORTON (1967a). On the other hand, acetylcholine might increase Na-, K-and Ca-permeabilities of the depolarized membrane (EVANS, SCHILD and THESLEFF, 1958;DURBIN and JENKINSON, 1961;SCHILD, 1964;JENKIN-SON and MORTON, 1967b). If the relaxation of the tonic response is not related to the ionic permeability of the membrane, isoprenaline might either reduce the free Ca ion in the myoplasm or inactivate the contractile process by some unknown means.…”

The Action of Adrenaline on the Ionic Content and on Sodium and Potassium Movements in the Smooth Muscle of the Guinea‐pig Taenia Coli

“…To produce complete depolarization, " K2SO0 Ringer" was used of the same composition as that used by Durbin & Jenkinson (1961)-that is, it contained (mM) K2SO4 76, KHCO3 16, CaCL2 7.5, Na2SO, 2.5, MgCl2 1.15, NaHPO, 0.5, glucose 6. To produce partial depolarization, "High KCI Ringer" was used in which the K-concentration of the Krebs solution was increased 10 times, to 59 mM, while the Na-concentration was reduced to 84.3 mM.…”