A cardioprotective agent of a novel calpain inhibitor, SNJ-1945, exerts beta1 actions on left ventricular mechanical work and energetics

Methods for the Preparation of an Excised, Cross-Circulated Rat Heart

The Emax-Pressure-Volume Area (PVA)-VO₂ framework proposed by Dr. Suga for canine hearts has dramatically advanced the field of cardiac mechanical work and energetics, i.e., mechanoenergetics. He and his collaborators investigated mechanoenergetics in the left ventricle (LV) of excised, cross-circulated canine heart preparations. We instituted the excised cross-circulated rat whole heart preparations and found a curvilinear end-systolic pressure-volume relation (ESPVR) in the rat LV, in contrast to the linear ESPVR in canine, rabbit, and human LVs. Although Emax, the slope of the linear ESPVR, could be used as an index of LV contractility, it was not applicable for evaluating LV contractility in the rat LV. Thus, we proposed a new index of contractility, equivalent Emax (eEmax) in the rat LV. We also found a linear VO₂-PVA relationship in the rat LV. Here, we introduce the methods for the preparation of excised, cross-circulated rat whole hearts and the eEmax-PVA-VO₂ framework in the rat LV. Using this method, we can obtain accurate LV volume and myocardial O₂ consumption in real time for estimating cardiac mechanoenergetics, which is very challenging in in vivo experiments.

Left ventricular mechanoenergetics in excised, cross-circulated rat hearts under hypo-, normo-, and hyperthermic conditions

We investigated the effects of altering cardiac temperature on left ventricular (LV) myocardial mechanical work and energetics using the excised, cross-circulated rat heart model. We analyzed the LV end-systolic pressure-volume relationship (ESPVR) and linear relationship between myocardial oxygen consumption per beat (VO2) and systolic pressure-volume area (PVA; total mechanical energy per beat) in isovolumically contracting rat hearts during hypo- (32 °C), normo- (37 °C), and hyperthermia (42 °C) under a 300-beats per minute pacing. LV ESPVR shifted downward with increasing cardiac temperature. The VO2-PVA relationship was superimposable in these different thermal conditions; however, each data point of VO2-PVA shifted left-downward during increasing cardiac temperature on the superimposable VO2-PVA relationship line. VO2 for Ca2+ handling in excitation-contraction coupling decreased, which was associated with increasing cardiac temperature, during which sarcoplasmic reticulum Ca2+-ATPase (SERCA) activity was suppressed, due to phospholamban phosphorylation inhibition, and instead, O2 consumption for basal metabolism was increased. The O2 cost of LV contractility for Ca2+ also increased with increasing cardiac temperature. Logistic time constants evaluating LV relaxation time were significantly shortened with increasing cardiac temperature related to the acceleration of the detachment in cross-bridge (CB) cycling, indicating increased myosin ATPase activity. The results suggested that increasing cardiac temperature induced a negative inotropic action related to SERCA activity suppression in Ca2+ handling and increased myosin ATPase activity in CB cycling. We concluded that thermal intervention could modulate cardiac inotropism by changing CB cycling, Ca2+ handling, and basal metabolism in rat hearts.

A new calpain inhibitor protects left ventricular dysfunction induced by mild ischemia-reperfusion in in situ rat hearts

We have previously indicated that a new soluble calpain inhibitor, SNJ-1945 (SNJ), attenuates cardiac dysfunction after cardioplegia arrest-reperfusion by inhibiting the proteolysis of α-fodrin in in vitro study. Nevertheless, the in vivo study design is indispensable to explore realistic therapeutic approaches for clinical use. The aim of the present in situ study was to investigate whether SNJ attenuated left ventricular (LV) dysfunction (stunning) after mild ischemic-reperfusion (mI-R) in rat hearts. SNJ (60 μmol/l, 5 ml i.p.) was injected 30 min before gradual and partial coronary occlusion at proximal left anterior descending artery. To investigate LV function, we obtained curvilinear end-systolic pressure-volume relationship by increasing afterload 60 min after reperfusion. In the mI-R group, specific LV functional indices at midrange LV volume (mLVV), end-systolic pressure (ESP(mLVV)), and pressure-volume area (PVA(mLVV): a total mechanical energy per beat, linearly related to oxygen consumption) significantly decreased, but SNJ reversed these decreases to time control level. Furthermore, SNJ prevented the α-fodrin degradation and attenuated degradation of Ca(2+) handling proteins after mI-R. Our results indicate that improvements in LV function following mI-R injury are associated with inhibition of the proteolysis of α-fodrin in in situ rat hearts. In conclusion, SNJ should be a promising tool to protect the heart from the stunning.

Calpain, not caspase, is the causative protease for hypoxic damage in cultured monkey retinal cells

PURPOSE

Cell death occurring in human retina during AMD, high IOP, and diabetic retinopathy could be caused by activation of calpain or caspase proteolytic enzymes. The purpose of the present study was to determine whether calpains and/or caspase-3 were involved in cell death during retinal hypoxia in a monkey model.

METHODS

Dissociated monkey retinal cells were cultured for two weeks and subjected to 24-hour hypoxia/24-hour reoxygenation. TUNEL staining and immunostaining for Müller and photoreceptor markers were used to detect which retinal cell types were damaged.

RESULTS

Culturing dissociated monkey retina cells for two weeks resulted in proliferation of Müller cells and maintenance of some rod and cone photoreceptor cells, as identified by vimentin, recoverin, and rhodopsin immunocytochemical staining. Hypoxia/reoxygenation increased the number of cells staining positive for TUNEL. Immunoblotting showed that the calpain-specific 145 kDa α-spectrin breakdown product (SBDP) increased in hypoxic cells, but no caspase-specific 120 kDa α-spectrin breakdown product was detected. TUNEL staining and proteolysis were significantly reduced in the retinal cells treated with 10 and 100 μM calpain inhibitor SNJ-1945. Caspase inhibitor, z-VAD, did not inhibit cell damage from hypoxia/reoxygenation. Intact pro-caspase-3 was in fact cleaved by activated calpain during hypoxia/reoxygenation to pre 29 kDa caspase-3 and 24 kDa inactive fragments. No 17 and 12 kDa fragments, which form the active caspase-3 hetero-dimer, were detected. Calpain-induced cleavage of caspase was inhibited by SNJ-1945.

CONCLUSIONS

Calpain, not caspase-3, was involved in hypoxic damage in cultured monkey retinal cells.