Complex interactions of agonists with alpha 1-adrenoceptors in intact cells

1. The apparent Ki values of (-)-noradrenaline (NA), (+)- and (-)-adrenaline (Ad), phenylephrine and the mono-fluorinated NAs (in position 2, 5 or 6) for alpha 1-adrenoceptors of intact BC3H1 cells labelled with [3H]-prazosin were greatly dependent on the incubation temperature. 2. The EC50 values of these compounds for stimulation of the inositol phosphate (IP) accumulation at 37 degrees C were intermediate between their apparent dissociation constants at 2 degrees C (Ki2 degrees) and at 37 degrees C (Ki37 degrees). 3. The fact that an irreversible blockade of 46% +/- 6% (n = 3) of the [3H]-prazosin binding sites by phenoxybenzamine reduced the maximal IP-formation induced by NA by 57% +/- 5% (n = 3) shows that there is a direct coupling between alpha 1-adrenoceptors and phospholipase C in BC3H1 cells. 4. The Ki37 degrees s of all agonists tested were in the same range (0.1 to 1 mM) and showed no simple correlation with their EC50 values. 5. The Ki2 degrees values for all the agonist correlated linearly with their EC50 values but were about 20-100 times lower than the respective EC50 values (except for the partial agonist methoxamine). In order to explain this difference, we propose that the apparent high affinity in the cold could be due to an [3H]-prazosin-induced alteration of the active site of the alpha 1-adrenoceptor, increasing its apparent affinity for catecholamines.