Diazepam does not improve the mechanical performance of rat cardiac papillary muscle exposed to chloroquine in vitro

Diazepam has been reported to decrease the cardiac toxicity of chloroquine but the precise mechanism involved remains unknown. Left ventricular papillary muscles from adult Wistar rats were exposed to 10^-4 M chloroquine and assigned to three groups: group I (n=10) exposed to chloroquine alone; group II (n=8) exposed to chloroquine and 10^-5 M diazepam; group III (n=8) exposed to chloroquine and 10^-4 M diazepam. The main mechanical parameters measured were: maximum unloaded shortening velocity (Vmax), maximum lengthening velocity (maxVr), active force normalized per cross-sectional area (AF/s), contraction-relaxation coupling under low load (R1), load sensitivity of relaxation (Isot.A/ Isom.A), and peak power output ( \(\mathop {\text{E}}\limits^{\text{o}} \) max) determined from Hill's equation of the force-velocity curve. Data are expressed as mean percent of control values±SD, for groups I, II, III respectively. No differences between groups I, II, and III were noted for Vmax (87±13, 82±9, 86±7), maxVr (47±6, 48±11, 52±11), AF/s (87±16, 91±10, 83±11), Isot. A/Isom. A (113±9, 108±3, 109±7), or \(\mathop {\text{E}}\limits^{\text{o}} \) max (75±10, 81±12, 72±16). Chloroquine was shown to be a negative inotropic agent since it decreased Vmax, AF/s and \(\mathop {\text{E}}\limits^{\text{o}} \) max, but diazepam did not restore the intrinsic mechanical performance of rat cardiac papillary muscle exposed to chloroquine, therefore 1) the protective cardiovascular effects of diazepam in chloroquine poisoning are not related to an improvement in intrinsic cardiac mechanical properties; 2) inotropic agents are therefore necessary in combination with diazepam for the treatment of severe chloroquine poisoning.