Effects of elastase on contractility and morphology of elastic tissue in isolated guinea pig papillary muscles

Elastase (ELA) is an enzyme catalyzing the digestion of elastin, an essential constituent of elastic fibers. Using isolated guinea pig papillary muscles, we examined the effect of ELA (3 x 10(-7) -3 x 10(-4) g/ml) on resting tension (RT) and twitch tension (TT). The effects of ELA on elastic fibers located in the subendocardium were examined histologically. A relatively high concentration of ELA (3 x 10(-4) g/ml) increased TT transiently, with progressive decreases in RT. In contrast, a relatively low concentration (3 x 10(-5) g/ml) decreased both TT and RT straightforwardly. Much lower concentrations (3 x 10(-6) -3 x 10(-7) g/ml) did not reveal significant effects. The ELA-induced increases in TT were unaffected in the presence of atenolol (10(-5) g/ml), ouabain (10(-7) M) or ryanodine (10(-6)M). ELA did not increase the maximum rate of rise of slow action potentials recorded using standard microelectrodes. ELA (3 x 10(-5) g/ml) decreased the maximum TT obtained at optimal RT or Lmax, and decreased the slope of the ascending and descending limbs of the TT-RT relation curve (Frank-Starling's). Electron-microscopic findings revealed that subendocardial elastin was mostly digested at ELA concentrations of 3 x 10(-5) -3 x 10(-4) g/ml. These findings suggest that the decrease of RT by ELA may be, at least in part, caused by a decomposition of the elastic fibers. On the other hand, the increase of TT by ELA could not be attributed to a release of endogenous catecholamine, an inhibition of Na+, K(+)-pump, a release of Ca2+ from sarcoplasmic reticulum, or an increase of slow inward current.