EVIDENCE FOR MULTIPLE SOURCES OF CALCIUM FOR ACTIVATION OF THE CONTRACTILE MECHANISM OF GUINEA‐PIG taenia coli ON STIMULATION WITH CARBACHOL

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Abstract
1 The evidence presented suggests there are three sources of Ca available for contraction of the smooth muscle of the guinea-pig taenia coli on stimulation with carbachol; the inward Ca current of the spike, a second voltage-dependent Ca channel and an internal Ca store. 2 The initial increment of tension in response to carbachol is thought to be due to an increase in spike frequency which is probably the main source of Ca at low carbachol concentrations (< 10−6 m). 3 The maintained tension in the continuous presence of high concentrations of carbachol seems to involve continuous influx of membrane-bound Ca by a potential-dependent mechanism which can be very quickly deactivated, resulting in rapid relaxation. This mechanism can be blocked by 2 × 10−7 m methoxyverapamil (D600). 4 An internal Ca store can be released by high concentrations of carbachol (−6 m) and is probably responsible for the initial transient peak of tension, of about 5 min duration seen on continuous application of high concentrations of carbachol and for the tension increase in response to carbachol in tissues depolarized in high-K. 5 Investigation of the properties of the store indicates that it; (i) is very rapidly filled by application of high extracellular Ca; (ii) empties after a few minutes in zero-Ca EGTA Krebs solution; (iii) can be refilled in depolarized tissues in the presence of low concentrations of D600 and Mn, but does not refill during application of carbachol at concentrations greater than 10−6 m; (iv) contains enough Ca for one near-maximal contraction and once emptied can assist relaxation by Ca re-uptake.