Increased oxyhemoglobin affinity by carbamylation: coronary autoregulation and O2 transport

Hemoglobin carbamylation with sodium cyanate was used to study myocardial O2 delivery during increased oxyhemoglobin affinity. A group of control dogs was compared with a group of dogs, in which their hemoglobin had been carbamylated by daily ingestion of sodium cyanate over a period of 6-8 wk. The O2 tension at 50% saturation of hemoglobin (P50) averaged 23 mmHg in the carbamylated group compared with an average P50 of 31 mmHg in the control group. The left main coronary artery was cannulated and perfused via an extracorporeal, pressure-controlled reservoir. Coronary pressure, coronary flow, and blood gas measurements were made at coronary perfusion pressures spanning the autoregulatory range. The increased oxyhemoglobin affinity produced no significant difference in convective O2 transport to the coronary capillaries, although the carbamylated group had a significantly higher arterial O2 content and significantly lower coronary flow. The autoregulatory process was not significantly altered by carbamylation-induced shifts in the O2 dissociation curve. Despite a significantly lower coronary sinus PO2 in the carbamylated group, the increase in oxyhemoglobin affinity was associated with significantly lower O2 extractions in the carbamylated group. Significant extraction reserve was present in both the control and carbamylated groups as demonstrated by the fact that O2 extraction could still increase well above control levels when perfusion pressure was lowered. We conclude that there is no significant vasodilatory compensation for moderate increases in oxyhemoglobin affinity, despite the continued presence of autoregulatory vasodilatory reserve.