Effect of low extracellular calcium on shortening velocity in isolated single smooth muscle cells

To understand how smooth muscle modulates its shortening velocity within the time course of a single contraction, the factors that govern crossbridge cycling in smooth muscle must be characterized. Since calcium plays an important role in regulation of the contractile apparatus, we studied the effect of lowering extracellular calcium on shortening velocity in single smooth muscle cells isolated enzymatically from the toad, Bufo marinus, stomach muscularis. Shortening velocity was estimated by three independent methods: (1) isotonic releases; (2) slack test; (3) video images of freely shortening cells. To determine the shortening velocity from isotonic releases and the slack test method, a single cell was attached at one end to an ultrasensitive force transducer and piezoelectric displacement device at its other end. At peak isometric force, in response to electrical stimulation, a series of isotonic releases to varying force levels were imposed and the resultant shortening responses used to construct force:velocity relationships in physiological saline containing low (0.18 mM) extracellular calcium. From force:velocity relationships and slack test data, an estimate of maximum shortening velocity (Vmax) was 0.19 cell lengths/s which was 3 times lower than in normal calcium (Warshaw 1987). Although Vmax was reduced in low extracellular calcium, the curvature of the force:velocity relationship estimated from the hyperbolic constant, a/Fmax, was unchanged (a/Fmax = 0.17). Additional evidence for an effect of extracellular calcium on shortening velocity was obtained from shortening responses of freely shortening cells attached at one end only. Data from cells exposed first to low (0.18 mM) and then normal (1.8 mM) calcium suggest at least a twofold decrease in free shortening velocity with lower extracellular calcium.(ABSTRACT TRUNCATED AT 250 WORDS)