Protective effect of amiloride during hypothermic hyperkalemic preservation: a 31P NMR study in isolated pig hearts

To assess the possible role of Na+/H+ exchange in ischemia-reflow damage during cold preservation, pig hearts retrogradely perfused with Krebs-Henseleit (KH) bicarbonate buffer were arrested by increasing [K+] (to 17 mM), cooled (10 degrees C) and subjected to no flow ischemia for 15 h in the absence (C, n = 5) and in the presence of 0.5 mM amiloride (A, n = 5). A was added 7 min prior to ischemia and removed after 3 min reflow. 31P nuclear magnetic resonance (NMR) spectra were continuously acquired and functional indices were assessed prior to and after ischemia. Before reflow, the levels of phosphocreatine (PCr), ATP, inorganic phosphate (Pi)+phosphomonoesters (PME) and cytosolic pH in C and A did not differ: C, 12 +/- 7, 56 +/- 13, 427 +/- 75% of initial and 5.90 +/- 0.20; A, 15 +/- 4, 82 +/- 21, 371 +/- 60% and 5.90 +/- 0.19, respectively; no contracture occurred. Upon reflow with hyperkalemic buffer PCr recovery in A-treated hearts was greater than in C (73 +/- 21 v 49 +/- 13%, P < 0.05) and parallel increases in left ventricular end diastolic (LVEDP) and perfusion pressure took place in both groups, however these parameters remained lower in A-treated hearts. Upon switching to KH buffer the oxygen uptake rate (VO2) was higher in A than in C (79 +/- 18 v 59 +/- 6%, P < 0.05) whereas the differences in the pressure-rate product (PRP, 51 +/- 21 in A v 38 +/- 11% in C) and LVEDP (16 +/- 11 in A v 24 +/- 7 mmHg in C) did not reach statistically significant levels. Adjusting LVEDP in both groups to the same levels (19 +/- 6 v 21 +/- 9 mmHg in C) by changing balloon volume resulted in significant difference in PRP (61 +/- 19% v 42 +/- 6% in C, P < 0.02). The coronary resistance measured in beating hearts remained lower in A than in C (34.5 +/- 3.0 v 57 +/- 11 mmHg/(ml/min g), P = 0.04). To estimate contractile and metabolic reserves post-ischemic heart were challenged by increasing [Ca2+] in the perfusate from 1.0 to 1.6 mM. PRP and VO2 increased; PRP reached 71 +/- 32 (A) and 54 +/- 10% (C) and VO2 remained higher in A: 90 +/- 16 (A) v 62 +/- 4% (C) (P = 0.01) of preischemic levels. We suggest that Na+/H+ exchange may contribute to ischemia-reflow damage during cold preservation of isolated pig hearts.