Mechanisms of positive inotropic effects and delayed relaxation produced by DPI 201–106 in mammalian working myocardium: effects on intracellular calcium handling

1 We used the bioluminescent protein aequorin, which emits light when it combines with Ca2+, to test the hypothesis that the inotropic and lusitropic actions of DPI 201–106 are due to changes in intracellular Ca2+ handling in papillary muscles from ferrets and guinea‐pigs. 2 DPI 201–106 increased peak isometric tension (T) in a dose‐dependent manner, with an 83% increase in T as the concentration of DPI 201–106 was increased to 1 × 10−5 m; however, peak [Ca2+]i did not increase significantly until the concentration of DPI 201–106 reached 3 × 10−6 m, suggesting a sensitization of the contractile apparatus to Ca2+. 3 Tetrodotoxin (1 × 10−6m), which did not reduce the tension response significantly before DPI 201–106, decreased both [Ca2+]i and T in the presence of 1 × 10−5m DPI 201–106, suggesting involvement of a sodium channel activation mechanism; however, tetrodotoxin did not completely reverse the calcium sensitization. 4 The shift of the [Ca2+]i versus T relationship was not observed in the presence of another sodium channel agonist, veratridine (3 × 10−7‐1 × 10−6m). 5 In the guinea‐pig, DPI 201–106 markedly prolonged relaxation of tension (increase of 60% in the time from peak to 50% tension regression), which was accompanied by the appearance of a second component in the aequorin light signal; effects on relaxation were less prominent in the ferret. 6 Tension prolongation and the second component of the [Ca2+]i transient in the guinea‐pig were exacerbated by increased [Ca2+]o and decreased by tetrodotoxin. Ryanodine (3 × 10−7 m) markedly diminished the calcium transient in controls and the initial component of the calcium transient in the presence of DPI 201–106, but had only a modest effect on the second component. 7 We conclude that although sodium agonism plays a role, sensitization of the contractile apparatus to Ca2+ is an important mechanism in the positive inotropic action of DPI 201–106. 8 The negative lusitropic action of DPI 201–106 varies between ferret and guinea‐pig, possibly reflecting differences between these two species in subcellular Ca2+ handling.