The myocardial tissue was sufficiently protected against possible extracellular acidosis resulting from successive anoxia-reoxygenation cycles. Indeed, taking into consideration the buffering capacity of the culture medium and the production of protons resulting from the metabolic activity of the whole heart at stage 20, ie, about 6 nmol O2/h , the external pH would decrease by only 0.005 unit within 1 h under stop-flow conditions. In addition, carbon dioxide cannot accumulate because there is no trace of carbon dioxide in the gas used in this study, and carbon dioxide produced by the embryonic myocardium diffuses rapidly and is washed out by the constant flux of gas. On the contrary, in ischemic isolated neonatal and adult hearts, regional accumulation of carbon dioxide cannot be avoided, specially in the case of metabolic acidosis when carbon dioxide is formed from bicarbonate.
A distinctive feature of our preparation is that all parts of the heart are simultaneously submitted to identical variations of PO2, whereas in isolated perfused hearts global ischemia is not homogeneous. Get most advantageous deals ever – actos for diabetes waiting for you round the clock.
In vivo, the nonvascularized embryonic myocardium is nourished mainly by blood pumped through the heart chambers, whereas in our preparation substrates and oxygen requirements were met exclusively by diffusion from the culture medium. However, exchanges with the outside environment are facilitated by the small size of the heart, the thinness of the myocardial wall and the contractions that stir the medium locally. Under normoxic conditions, PO2 values (approximately 6.7 kPa) in the deepest myocardial layers were even higher than those (approximately 1.3 kPa) found in vivo in the embryonic tissue at day 4 .