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 x 10(-5) M; however, peak [Ca2+]i did not increase significantly until the concentration of DPI 201-106 reached 3 x 10(-6) M, suggesting a sensitization of the contractile apparatus to Ca2+. 3. Tetrodotoxin (1 x 10(-6) M), which did not reduce the tension response significantly before DPI 201-106, decreased both [Ca2+]i and T in the presence of 1 x 10(-5) M 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 x 10(-7)-1 x 10(-6) M). 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 x 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.
Kihara
Y
Y
Charles A. Dana Research Institute, Boston, Massachusetts.
Gwathmey
J K
JK
Grossman
W
W
Morgan
J P
JP
D000331:Aequorin / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002217:Carbachol / Q000494:pharmacology; D005289:Ferrets; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects*; D066298:In Vitro Techniques; D008297:Male; D009126:Muscle Relaxation / Q000187:drug effects; D009200:Myocardial Contraction / Q000187:drug effects*; D009206:Myocardium / Q000378:metabolism*; D010210:Papillary Muscles / Q000187:drug effects / Q000378:metabolism; D010879:Piperazines / Q000494:pharmacology*; D012433:Ryanodine / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology; D014701:Veratridine / Q000494:pharmacology
D010879:Piperazines; D012433:Ryanodine; D013779:Tetrodotoxin; D000331:Aequorin; D014701:Veratridine; D002217:Carbachol; C046261:DPI 201-106; D002118:Calcium