The present work investigates the effects of acid-base alterations upo
n myocardial relaxation. Experiments were performed in cat papillary m
uscles contracting isometrically at constant frequency (0.2 Hz) and te
mperature (29-degrees-C). To induce intracellular alkalosis at constan
t pH0, 20 mM NH4Cl were added to the perfusate. Alkalosis at variable
pH0 was induced by switching from the control solution (5% CO2 -95% O2
, pH0 7.40) to a solution identical to the control one, equilibrated w
ith 3% CO2 -97% O2. Acidosis was induced by switching the control perf
usate to a solution equilibrated with 12% CO2 -88% O2 in which pH0 was
either allowed to change or kept constant by manipulation of bicarbon
ate concentration. Alkalosis produced a negative lusitropic effect eit
her when pH0 was kept constant or when it was allowed to increase. For
an increase in myocardial contractility of 30%, half relaxation time
(T50) and time to peak tension (TTP) were prolonged 9.4 +/- 5% and 5.4
+/- 2% respectively at constant pH0 and 6.8 +/- 0.8 and 4.7 +/- 1% re
spectively at variable pH0. It is suggested that this negative lusitro
pic effect of alkalosis can be attributed to an increase in myofilamen
t sensitivity to calcium. Either at constant or at variable pH0 acidos
is decreased myocardial contractility by approximately 50%. This decre
ase in contractility was accompanied by a positive lusitropic action o
nly when pH0 was allowed to decrease, or when acidosis at constant pH0
was evoked in the presence of EIPA, a specific inhibitor of the Na+/H
+ exchanger. The results indicate that the positive inotropic effect o
f a mild hypercapnic acidosis can be detected only if the degree of in
tracellular acidosis was enhanced after inhibition of the Na+/H+ excha
nger. Under these conditions, a decrease in myofilament sensitivity to
calcium can account for the positive lusitropic effect observed.