Chemosensitivity of cardiac muscle.

1. Measurements of contractions, conduction velocity and intracellular potential were made on isolated rabbit atria under four sets of conditions: high bicarbonate/high CO(2) (HH), low bicarbonate/low CO(2) (LL), high bicarbonate/low CO(2) (HL) and low bicarbonate/high CO(2) (LH). The ratio high/low was the same for the bicarbonate and CO(2) concentrations, so that HH had the same pH as LL.2. Acid solutions caused a fall of a few mV in the resting potential, but not in the overshoot. They reduced conduction velocity and rate of rise of the action potential. They depressed contractions, but prolonged the tail of the action potential.3. Alkaline solutions caused the converse changes, but, with the exception of the effect on the duration of the action potential, the relation with pH was markedly alinear, in that a rise in pH had much less effect than an equivalent fall.4. Statistical tests were devised to decide whether the observed changes were associated primarily with pH, P(CO(2) ) or bicarbonate. By far the strongest association was with external pH. Changes in P(CO(2) ), per se, had no significant effect.