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Studies of the roles of calcium ions in anterior mesenteric portal vein Collins , Glenn Albert

Abstract

There are two opposing schools of thought concerning the source of calcium ions for the initiation and maintenance of contractions of smooth muscle. Bohr (1964) and Woodward et al. (1970) believe that the calcium for the initiation of contraction is released from bound intracellular stores, whereas Somlyo et al. (1969) believe that most of the activator calcium comes from the extracellular fluid. It was felt that the determination of the source(s) and sink(s) of calcium ions in arteriolar smooth muscle would be required for an understanding of the control of peripheral blood pressure, and so experiments to obtain this information were carried out using the rabbit anterior mesenteric portal vein as a model of arteriolar smooth muscle. Spontaneous contractions of the vein stop within one minute after the addition of EGTA to the bath and can be returned by simply raising the extracellular calcium concentration. The addition of 1 mM MnC1₂, or raising the MgC1₂ concentration above 5 mM, or raising the CaC1₂ concentration above 10 mM all inhibit spontaneous activity; this inhibition can be rapidly reversed by the addition of appropriate amounts of EGTA. The removal of free extracellular calcium either by adding EGTA or placing the vein in calcium-free solution inhibits the responses to all agonists within five minutes. If the calcium concentration is reduced from 2.5 mM to 0.1 mM, the responses to agonists are greatly decreased. If one sets the response in normal Kreb's solution to each concentration of agonist equal to 100 per cent, then the relative reduction of responses in low calcium solution is inversely proportional to both the potency and concentration of the agonist used. If, however, one produces contractions by adding calcium to tissues bathed in calcium-free solution containing noradrenaline, then the curves of the relative response versus calcium concentrations are independent of the concentration of noradrenaline. The addition of either EGTA or manganese to a tissue already contracted in response to any agonist produces a rapid relaxation to a decreased, but sustained tension. The degree of relaxation is proportional to the concentration of manganese or EGTA added. The addition of manganese is also able to inhibit the initiation of responses to noradrenaline, KC1, serotonin, histamine and procaine. The inhibition by manganese of the responses to noradrenaline, KC1, and serotonin but not histamine and procaine can be reversed by increasing the extracellular calcium concentration. The addition of MnC1₂ or LaCl₃ does not selectively inhibit a slow phase of the contraction to noradrenaline in the mesenteric portal vein as it does in aorta (Van Breeman, 1969). The effect of adding MnCl₂ is similar to the effect of decreasing extracellular calcium, in that the relative inhibition of response is inversely proportional to the potency and concentration of agonist used. The effect of altering pH is the same on contractions produced by each agonist tested; lowering the pH below 7.4 inhibits the responses, raising it above pH 7.4 potentiates the responses. If the vein is placed in calcium-free solution containing EGTA for 10 minutes, the addition of CaC1₂ produces a contraction. The response to calcium is transient unless the final concentration of the added calcium is 5 mM or greater; at these higher calcium concentrations the response is biphasic; an initial transient response is followed by a slow tonic response. The addition of stock solutions to produce final concentrations of 5 mM Mg⁺⁺ or 0.5 mM Mn⁺⁺ or 0.1 mM Ca⁺⁺ in the bath after the EGTA treatment abolishes the transient responses to calcium but has little effect on the tonic portion of a contraction produced by 10 mM Ca⁺⁺. When these EGTA experiments are carried out in low sodium solution (17% of normal or less) the response to 2.5 mM Ca⁺⁺ which is normally transient, becomes instead a sustained contraction. It is concluded then that: 1. Calcium ions themselves are able to control the permeability of the membrane to calcium. 2. The initiation of responses to all agonists probably involves the release of membrane-bound calcium and the influx of extracellular calcium. Differences in efficacy are probably due to differences in ability to release the membrane-bound calcium. 3. A continued influx of calcium is required to maintain a contraction produced by any agonist. 4. Manganese competes with calcium at a membrane site to inhibit the initiation of contraction, and to relax a tissue which is already contracted. 5. Responses to all agonists are potentiated at a pH greater than 7.4, and decreased below pH 7.4. 6. The relaxation process in the mesenteric-portal vein seems to depend in some manner upon extracellular sodium ions.

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