| Abstract: | We have studied beta-adrenergic stimulation of cyclic AMP formation in fragmented membranes and in unsealed or resealed ghosts prepared from rat reticulocytes. The maximal rate of isoprenaline-stimulated cyclic AMP formation with saturating MgATP concentrations and in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine was 5-8 nmol/min per ml ghosts and remained constant for at least 15 min. Transition from resealed ghosts to fragmented membranes was associated with a shift of the activation constant (Ka) for (+/-)-isoprenaline from 0.1 to 0.6 microM. THe apparent dissociation constant for propranolol (0.01 microM) remained unchanged. The Ka values for isoprenaline in native reticulocytes and in resealed ghosts were identical. The stimulating effect of NaF on cyclic AMP formation in resealed ghosts reached 15% of maximal beta-adrenergic stimulation. Cyclic AMP formation, both in fragmented membranes and in ghosts, was half-maximally inhibited with Ca2+ concentrations ranging between 0.1 and 1 microM. GTP stimulated isoprenaline-dependent cyclic AMP formation in unsealed ghosts and in fragmented reticulocyte membranes by a factor of 3-5 but did not change the Ka value for isoprenaline. Ka values for the guanylnucleotides in different experiments varied between 0.3 and 2 microM. Ca2+ concentrations up to 4.6 microM reduced the maximal activation by GTP and Gpp(NH)p but did not affect their Ka values. Compared to GTP, maximal activation by Gpp(NH)p was higher in fragmented membranes, but much lower in ghosts. Our results suggest that the native beta-receptor adenylate cyclase system of reticulocytes is more closely approximated in the ghost model than in fragmented membrane preparations. Membrane properties seem to modulate the actions of guanylnucleotides on isoprenaline-dependent cyclic AMP formation in ghosts. Some of these effects are not observed in isolated membranes. |
