Fig. 6. Enhancement of the actions of bicarbonate by the phosphodiesterase inhibitor IBMX and blockade by the PKA inhibitor H89. (A) Enhancement by IBMX. Ca²⁺ channel activity was determined as in Fig. 1B, except that the third stimulus followed 1 minute perfusion by medium Na7.4 alone (none), with 15 mM NaHCO₃, or with 0.2 mM IBMX and NaHCO₃, as indicated. The depolarization-evoked rate of rise was normalized to the average of rates for the preceding two control stimuli. Mean values (n=13) were 0.95±0.10, 1.78±0.20 and 2.57±0.45. (B) Inhibition by H-89. In similar experiments, sperm in Na7.4 received a control stimulus, 1 minute of recovery in Na7.4 with 0, 1, 10 or 30 µM H89 then 2 minute exposures to Na7.4 with 15 mM NaHCO₃ and the same concentration of H89. The test stimulus used K8.6 containing the same concentration of H89. Normalized mean values (n=11) were 6.64±0.74, 4.39±0.37, 3.85±0.82 and 2.96±0.39. (C,D) Beat frequencies for cells randomly selected after 5-15 minutes in Na7.4 with: (C) 0, 50, 100 or 200 µM IBMX; (D) no additions, 30 µM H89, 15 mM NaHCO₃ (BC), or 30 µM H89 (5 minutes preincubation) and 15 mM NaHCO₃ as indicated. For each averaged point in C, n=15 cells from three independent experiments. For the data groups in D, n=41-43 cells from three independent experiments. Boxes indicate ±1 s.d. from the mean.