3H-mepyramine binding and histamine-stimulated cAMP accumulation in mammalian retina

The specific binding of different amounts of 3H-mepyramine to the bovine retina revealed a quasi-hyperbolic curve which approached saturation at 3H-ligand concentration over 9-12 nM. Scatchard analysis of the binding data showed two binding sites with KD values of 0.76 nM and 7.3 nM and Bmax of 49.3 and 194.6 fmole/mg protein, respectively. In the guinea-pig brain 3H-mepyramine bound to a single population of binding sites with KD value of 1.6 nM and Bmax of 291 fmole/mg protein. Various H1-antihistamines were potent competitors of the 3H-mepyramine binding: there was a big difference in potency of d- and 1-chlorpheniramine in both membrane preparations. In the rabbit retina slices histamine, in contrast to dopamine, weakly stimulated cAMP accumulation. The data suggest that the mammalian retina may possess histamine receptors.