Excitation-contraction coupling model to estimate the recirculating fraction of activator calcium in intact cardiac muscle.
Potentiated contractions were evoked with rapid pace pause maneuver in 14 length-clamped ferret papillary muscles paced 12 times/min at 25 degrees C. At 1.25 mM [Ca2+]o the average steady-state force was 2.94 +/- 1.08 g/mm2 and the potentiated contraction averaged 10.96 +/- 1.61 g/mm2. At 5.0 mM [Ca2+]o the steady-state force increased to 6.18 +/- 1.23 g/mm2 and the potentiated contraction averaged 12.08 +/- 1.15 g/mm2. Under the conditions of these experiments the potentiated contraction obtained at 5.0 mM [Ca2+]o is equal to the maximum twitch tension (Fmax) these muscles can generate. We have previously shown that Fmax is an equivalent of maximal calcium activated force. Since there is a beat to beat nearly exponential decay of the evoked potentiation, the fraction (= fraction x) of the potentiation that is not dissipated with each beat is nearly constant. Using an excitation-contraction coupling model we have previously found that x reflects a measure of the recirculating fraction of activator calcium. Because the tension-calcium relationship is better characterized by a sigmoidal curve, we have now incorporated the Hill equation in the model. To account for the inverse relationship between [Ca2+]i and the magnitude of the slow inward current, a term for negative feedback (h) was also included. We have determined the quantity (x-h) because x and h could not be determined separately. The quantity (x-h) was denoted as x'. The average values of x' at 1.25 and 5.0 mM [Ca2+]o were significantly different (p less than 0.0001), approximately 20% at the lower [Ca2+]o and about 50% at the higher [Ca2+]o.(ABSTRACT TRUNCATED AT 250 WORDS)
Urthaler
F
F
Department of Medicine, University of Alabama, Birmingham 35294.
Walker
A A
AA
Reeves
R C
RC
Hefner
L L
LL
D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D005117:Cardiac Complexes, Premature / Q000503:physiopathology; D002395:Catecholamines / Q000494:pharmacology; D003600:Cytosol / Q000378:metabolism; D004558:Electric Stimulation; D004576:Electromyography; D005246:Feedback; D005289:Ferrets; D006321:Heart / Q000502:physiology*; D066298:In Vitro Techniques; D008954:Models, Biological; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000378:metabolism; D010210:Papillary Muscles / Q000378:metabolism / Q000502:physiology
D002395:Catecholamines; D000109:Acetylcholine; D002118:Calcium