Inotropic interventions and myocardial force-interval relation: a quantitative approach.
OBJECTIVE
To analyze inotropic influence on the early and late phases of cardiac sarcoplasmic reticulum calcium loading.


METHODS
Papillary muscles with parallel edges and no evidence of tissue branching were selected from the heart. Only muscle preparations that maintained stable passive diastolic and developed forces were used for analysis. Muscles were stretched to their maximum length and stimulated at 0.2 Hz. The early and late phases of sarcoplasmic reticulum calcium loading were evaluated quantitatively by mathematical fitting of the force-interval relation. Increasing the extracellular calcium or decreasing the extracellular sodium was used to increase the inotropic state.


METHODS
Right ventricular papillary muscles were isolated from female Wistar rats weighing 200 to 220 g. Electrical stimulation and data acquisition were controlled through a microcomputer.


RESULTS
Increasing the extracellular calcium concentration from 0.5 to 1.0 mM produced a 90% increase in developed tension. This was accounted for by a 41% increase in the early phase of sarcoplasmic reticulum loading and a 29% increase in the late phase. A 20% reduction in the extracellular sodium concentration increased contractile force 100% and shifted the force-interval curve to the left. This was accounted for through an increase in both early and late phases of sarcoplasmic reticulum loading.


CONCLUSIONS
These results are consistent with the current model of excitation-contraction coupling and clearly indicate that various positive inotropic interventions have selective effects on each process of the force-interval relation that cooperatively interact with each other. Mathematical fitting of data clearly improves the quantitative aspect of the force-interval response.
Department of Biological Sciences, University of Windsor, Ontario.
P B
PB
Taylor
R K
RK
Helbing
K A
KA
Kenno
D000818:Animals; D002118:Calcium / Q000494:pharmacology / Q000502:physiology; D005260:Female; D066298:In Vitro Techniques; D009200:Myocardial Contraction / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000494:pharmacology / Q000502:physiology
D012964:Sodium; D002118:Calcium