Control of shortening speed in single guinea-pig taenia coli smooth muscle cells by Ca2+, phosphorylation and caldesmon.
We studied the effect of caldesmon peptides on the regulation of shortening of single guinea-pig taenia coli cells permeabilised with saponin. When contraction was initiated by Ca2+ and MgATP shortening rate at pCa 4.5 was 0.17+/-0.04 cell lengths s-1 and half-maximal rate was at pCa 5.6. Following thiophosphorylation with 1 mM adenosine 5'-O-(3-thiotriphosphate) (ATP[gamma-S]) at pCa 4.5 for 10 min, on addition of ATP these cells contracted at of 0.25+/-0.04 cell lengths s-1 independently of pCa. If thiophosphorylated cells were preincubated with H1 (domains 3 and 4 of caldesmon), shortening speed was reduced (ID50=2 microM). Shortening speed was also reduced by 658C (domain 4b) at higher concentrations (ID50=400 microM). H13 (domain 4a), which does not block weak binding but inhibits actin-tropomyosin, inhibited cell shortening (ID50=6 microM). H2, which blocks weak binding but does not inhibit actin-tropomyosin, did not inhibit shortening. Western blots of the cells showed that the peptides were tightly bound within the cell but the native caldesmon was not displaced. These results indicate that exogenous caldesmon peptides added to smooth muscle cells may be incorporated into the thin filaments and produce effects on shortening, as expected if it were involved in tropomyosin-dependent inhibition of the actin filament in the cell.