Effects of low extracellular Ca2+ on prostaglandins F2 alpha and E2 action in longitudinal muscle of guinea-pig caecum.

1. Effects of low extracellular Ca2+ on the action of prostaglandin F2 alpha (PGF2 alpha) and prostaglandin E2 (PGE2) in longitudinal muscle of guinea-pig caecum were evaluated by the changes in the membrane potential, action potential frequency and contractions. 2. At 2.5 mmol extracellular Ca2+ the EC50 values for the stimulant effects of PGF2 alpha and PGE2 on the membrane potential, the frequency of action potential discharge and contractions were higher for PGF2 alpha than the PGE2. 3. The lowering of extracellular Ca2+ from 2.5 mmol to 0.4 mmol did not affect essentially the PGF2 alpha and PGE2 effects. 4. In Ca2+-free solution containing EGTA the stimulant effects of PGF2 alpha and PGE2 significantly decreased but the action potentials and contractions were not completely abolished. D-600 pretreatment inhibited in a dose-dependent manner the PGF2 alpha- and PGE2-evoked action potential discharges and contractions. 5. It is concluded that the mechanisms limiting Ca2+ influx at 2.5 mmol extracellular Ca2+ are more sensitive to PGE2 than to PGF2 alpha. 6. At low extracellular Ca2+ and in Ca2+-free solution containing EGTA the effects of PGF2 alpha and PGE2 on the action potential generation and the corresponding contractions might be determined by Ca2+-independent processes.