No abstract

The dopamine receptor in the intermediate lobe (IL) of the hypophysis of the rat is characterized on the basis of the ability of dopamine and other dopaminergic agonists to decrease the consequences of activation of the beta-adrenoceptor. Stimulation of the dopamine receptor diminishes the L-isoproterenol-induced accumulation of cAMP and the catecholamine-stimulated enhancement of adenylate cyclase activity. The dopamine receptor in the IL can be assigned to the category of dopamine receptor designated D-2 on the basis of the following criteria: 1) occupancy of the dopamine receptor does not result in enhancement of adenylate cyclase activity or an accumulation of cAMP, 2) the dopaminergic ergots and apomorphine mimic the inhibitory effect of dopamine upon cAMP formation or alpha MSH release, and 3) metoclopramide and sulpiride, substituted benzamides, block the inhibitory effect of dopamine. The sympathetic neurotransmitter, norepinephrine, interacts with the dopamine receptor and the beta-adrenoceptor in the IL.

The intermediate lobe (IL) of the rat pituitary gland responds to catecholamines. Catecholamines interacting with the β-adrenoceptor stimulate adenylate cyclase activity, enhance cyclic AMP formation and thereby trigger the release of α-melanocyte-stimulating hormone (α-MSH). Catecholamines interacting with a D-2 dopamine receptor (in the classification schema of Kebabian and Calne) diminish adenylate cyclase activity and thereby decrease the capacity of IL cells to synthesize cyclic AMP. Dopaminergic agonists also inhibit the release of α-MSH from IL cells. The homogeneity of the IL facilitates biochemical investigations of this tissue.

After treatment with cholera toxin, homogenates of intact intermediate lobe (IL) tissue of rat pituitary gland synthesized more cAMP than did homogenates of untreated IL tissue, and only in the presence of GTP did dopamine or apomorphine diminish the elevated adenylate cyclase activity in homogenates of cholera toxin-treated IL tissue. Furthermore, when tested on cholera toxin-treated IL tissue, 5'-guanylyl imidodiphosphate [Gpp(NH)p] and two other nonhydrolyzable analogs of GTP inhibited adenylate cyclase activity in the absence of either a dopaminergic agonist or GTP; GTP reversed the Gpp(NH)p-induced inhibition of adenylate cyclase activity. Apomorphine, a dopaminergic agonist, abolished the ability of GTP to reverse the inhibition by Gpp(NH)p; this effect of apomorphine was prevented by fluphenazine, a dopaminergic antagonist. Sodium fluoride inhibited adenylate cyclase activity to approximately the same level obtained with GTP and apomorphine. In addition, apomorphine decreased cAMP accumulation and alpha MSH release from dispersed IL cells pretreated with cholera toxin.

Stimulation of the release of alpha MSH from dispersed rat melanotrophs by L-isoproterenol, 8-bromo-cAMP, or cholera toxin requires calcium ion (Ca++) in the incubation medium; the stimulatory effect of each of these agents is attenuated by D-600, a Ca++ antagonist. In contrast, stimulation of the formation of cAMP by L-isoproterenol, isobutyl methylxanthine, or cholera toxin does not require Ca++ in the incubation medium. Ca++ diminishes the amount of cAMP formed by cholera toxin-treated melanotrophs. Ca++ inhibits adenylate cyclase activity and enhances cyclic nucleotide phosphodiesterase activity in cell-free homogenates of intermediate lobe tissue. A23187, a calcium ionophore, increases the accumulation of 45Ca by melanotrophs and enhances the release of alpha MSH. Furthermore, when tested upon cholera toxin-treated melanotrophs, A23187 potentiates the Ca++-induced inhibition of cAMP formation. The results indicate that Ca++ is essential for the release of alpha MSH, and that cAMP in some way enhances the effects of Ca++ upon the release process.