Hemodynamics of the orthotopic homotransplanted canine heart

Characterization of the natural history of cervical heterotopic heart transplantation with echocardiography

BACKGROUND

Limitations of the dog model of orthotopic heart transplantation to study rejection include the need for extracorporeal circulation and transfusions. Heterotopic cervical heart transplantation may improve on these limitations. It is not known whether the natural history after heterotopic transplant is similar to that after orthotopic grafting.

METHODS

Twenty-one dogs underwent cervical heterotopic heart transplantation. Serial echocardiographic studies were performed 1 to 3 hours after surgery, at 24 hours, 48 hours later, and immediately before killing (5 to 7 days).

RESULTS

LV diastolic and systolic areas were elevated immediately after transplantation (4.95+/- 1.49 cm2 and 3.36+/-1.18 cm2 respectively) but decreased at 24 hours (3.93+/-1.20 cm2, p = 0.0003 and 2.44+/-0.96 cm2, p = 0.16). Thereafter, a progressive increase in LV diastolic and systolic areas was observed until sacrifice (5.53+/-2.20 cm2 and 4.59 +/-2.14 cm2, p < 0.001 vs 24 hours). LV fractional area shortening (FAS) and fractional volume change were depressed immediately after transplantation (28.2+/- 12.8% and 40.4+/-12.3%, respectively), but increased at 24 hours (35.7+/-10.0%,p = 0.11 and 50.3+/-4.0%,p = 0.02). FAS decreased at 48 hours to 19.6+/-11.1% (p = 0.01 vs 24 hours). The centractility indexes were markedly reduced before killing (FAS = 14.0+/-8.2% and LVEF = 18.4+/-1.3%, p < 0.0005 vs 24 hours). The thickness of the interventricular septum increased from 11.9+/-2.0 mm at baseline to 14.4+/-4.2 mm before sacrifice (p = 0.007).

CONCLUSION

The evolution of dogs after heterotopic cervical heart transplant is comparable to that after the more standard orthotopic graft. Considering its multiple practical advantages including the easy echocardiographic follow-up, heterotopic transplantation may become a very practical model to use for the study of rejection after heart transplantation.

Mechanisms whereby cytotoxic T lymphocytes damage guinea-pig ventricular myocytes in vitro

We studied possible mechanisms whereby cytotoxic T lymphocytes (CTL) damage the myocardium during the immunological rejection of the transplanted heart, by investigating the in vitro interaction between CTL and cardiac myocytes. We utilized the patch-clamp technique to record membrane currents and action potentials from concanavalin A-treated guinea-pig ventricular myocytes conjugated to mouse peritoneal exudate CTL (PEL). PEL-myocyte interaction reduced action potential duration at 50% repolarization (APD50) from 731.7 +/- 57.8 to 195.3 +/- 58.0 ms, action potential amplitude from 134.9 +/- 1.9 to 104.2 +/- 6.2 mV and resting membrane potential (Vm) from -80.9 +/- 0.5 to 72.5 +/- 1.5 mV. These changes were accompanied by generation of delayed afterdepolarizations, indicative of intracellular [Ca2+] overload. The electrophysiological alterations were associated with myocyte shortening (within 28.9 +/- 2.8 min) followed by complete cell destruction (within 43.5 +/- 4.3 min). To determine whether intracellular Ca2+ stores were involved in PEL-induced myocyte damage, the protective effects of ryanodine and caffeine were investigated. While ryanodine (10 microM) delayed the electrophysiological and morphological alterations, caffeine (5 mM) provided significant protection, suggesting that Ca2+ release from intracellular stores contributes to PEL-induced damage to the myocytes. Based on our findings, we suggest that the functional derangements seen in myocyte-lymphocyte conjugates can contribute to the overall decline in cardiac function during heart transplant rejection.

Efficacy of isoproterenol on the failing transplanted heart during early acute rejection

Acute rejection often leads to severe myocardial failure and death. The beneficial hemodynamic effects of isoproterenol in improving immediate postoperative heart failure have prompted its routine use after transplantation. However, because of the physiopathological alterations documented during rejection, an inappropriate response of the graft to isoproterenol administration could be expected. Six dogs received orthotopic transplants and were prepared with implantable devices for serial hemodynamic studies. The studies were performed on the resting unanesthetized subject 3 hours after operation when transient heart failure was present and repeated when myocardial failure secondary to rejection occurred. After basal state measurement, various doses of isoproterenol were infused and the hemodynamic responses during each period were compared. During rejection, the hemodynamic response to 0.05 and 0.10 micrograms.kg-1.min-1 was significantly lower when compared with the response in the postoperative period. To achieve similar postoperative chronotropic and inotropic effects, 0.35 microgram.kg-1.min-1 of isoproterenol was necessary. Isoproterenol is therefore effective in controlling myocardial failure during acute rejection despite a reduced sensitivity of the sinoatrial node and myocardial tissue.

Decreased digitalis receptor activity in acute rejecting canine transplanted heart

In the course of acute rejection, myocardial tissue undergoes massive transformation and we hypothetized that for digitalis-like substances, receptor binding characteristics might be altered. Ten canine heterotopic cardiac allografts were carried out and were harvested once rejection had developed (8-10 days post-transplant). Microsomal membrane fractions of those grafts and of native hearts were isolated. Radioligand binding studies were carried out in a medium containing 5 mM Tris PO4, 50 mM Tris HCl, 5 mM MgCl2, pH 7.4 at 37 degrees C, using 3H-ouabain as the ligand. Saturation experiments (n = 10) indicate the presence of one homogeneous population of high affinity binding sites with an affinity constant (Kd) of 8-13 nM and a maximum binding capacity (Bmax) of 47 +/- 3.5 pmol/mg protein. Both saturation and competition binding studies illustrate the fact that acute rejection resulted in a significant decrease in Bmax (43%) without significant alteration in Kd value. These studies indicate that digitalis-like substances might not exert significant inotropic activity during rejection, but this hypothesis must be confirmed by in vivo haemodynamic experiments.