Effects of trimetazidine on pH(i) regulation in the rat isolated ventricular myocyte

1 We have examined the effects of trimetazidine (TMZ) on intracellular pH (pH(i)) regulation in rat isolated ventricular myocytes. pH(i) was recorded ratiometrically by use of the pH-sensitive fluoroprobe, carboxy-SNARF-1 (carboxy-seminaphtorhodafluor). 2 Following an intracellular acid load (induced by 10 mM NH4Cl removal), pH(i) recovery in HEPES-buffered Tyrode solution was significantly slowed down upon application of 0.3 mM TMZ only when myocytes were pretreated for 5 h 30 min (slowing by similar to 50%; P < 0.01). This effect of TMZ on pH(i) recovery was shown to be not only time- but also dose-dependent with a large, quickly reversible, effect obtained with 1 mM TMZ applied for 2-3 h (slowing by similar to 64%; P < 0.001). This slowing of pH(i) recovery was also associated with a decrease of the NH4+ removal-induced acidification. 3 Relationship between intracellular intrinsic buffering power (beta(i)) and pH(i) was assessed in absence or presence of TMZ (0.3 mM or 1 mM). As expected, beta(i) increased roughly linearly with a decrease in pH(i) in all cases. However, both concentrations of TMZ significantly increased beta(i) (by similar to 55 and 65% at pH(i) 7.1, respectively). 4 When Na+/H+ exchange was inhibited by dimethyl amiloride (DMA; 40 mu M), trimetazidine (1 mM) did not change the H+ flux estimated at pH(i) 7.1 (0.31 +/- 0.03 mequiv 1(-1) min(-1), n = 5, control, versus 0.30 +/- 0.025 mequiv 1(-1) min(-1), n = 5, TMZ), ruling out any effect of TMZ on background acid loading. 5 Acid efflux carried by Na+/H+ exchange was significantly decreased only when myocytes were pretreated with 1 mM TMZ, for 2-3 h (J(H)(e) = 2.86 +/- 0.38 mequiv 1(-1) min(-1), n = 26, control, versus 1.66 +/- 0.26 mequiv 1(-1) min(-1), n = 10, TMZ, estimated at pH(i) 7.1; P < 0.05). 6 In conclusion, the present work demonstrates that, following an intracellular acid load in HEPES-buffered medium, trimetazidine slows down pH(i) recovery in rat isolated ventricular myocytes, primarily through an increase of beta(i). An effect on Na+/H+ exchange is also detected but only after long-term incubation of the myocytes with TMZ.