The effects of hydrocortisone on responses to and extraneuronal uptake of (-)-isoprenaline in cat and guinea-pig atria.

1. (-)-Isoprenaline-induced positive inotropic effects were assessed in driven left atria and positive chronotropic effects in spontaneously beating right atria from guinea-pigs and cats. 2. Hydrocortisone produced dose dependent shifts to the left in cumulative concentration-effect curves to (-)-isoprenaline in left and right atrial preparations from the cat. 3. Maximal potentiations were produced by 160 muM and 40 muM hydrocortisone in cat left and right atria respectively. 4. No such effects were seen in similar preparations from the guinea-pig even in the presence of concentrations of hydrocortisone as high as 400 muM. 5. Hydrocortisone inhibited the extraneuronal uptake of 7 - 3H-isoprenaline in a concentration dependent manner in both left and right atria of the cat, but not the guinea-pig. 6. These results are consistent with the idea that hydrocortisone potentiates responses to (-)-isoprenaline in cat atria by inhibition of the extraneuronal uptake process.