Pars intermedia cell activity

Figure 8. Representation of the interaction between CRF, -adrenergic, and dopaminergic (DA.) receptors in the control of pars intermedia cell activity. The CRF and / -adrenergic receptors stimulate adenylate cyclase activity through interaction with the Ns-GTP-binding component. Dopamine, on the other hand, interacts with the Ni-GTP-binding component, causing inhibition of basal as well as CRF- and f3-adrenergic-induced adenylate cyclase activity.

In rat pars intermedia cells, the rate of secretion of the proopiomelanocortin-derived peptides (a-MSH being the major secretory product) (3, 85) was so far known to result from a balance between the stimulatory effect of B-adrenergic agents and the inhibitory influence of dopaminergic substances (14, 41, 86-89). The present data clearly demonstrate that in addition to B-adrenergic agents, a second substance, namely CRF, could well be involved as physiological stimulator of the activity of pars intermedia cells. 

The above-described data show that CRF added to cells of the rat Intermediate lobe In culture causes a rapid stimulation of oe-MSH release and cyclic AMP accumulation, thus demonstrating a direct action of the peptide on pars intermedia cells (15). It is however difficult, using intact cells, to dissociate between increases in cyclic AMP levels due to stimulation of adenylate cyclase activity or to Inhibition of cyclic nucleotide phosphodiesterase or to a combination of both effects. Definitive proof of the role of adenylate cyclase In the action of CRF In the intermediate lobe of the pituitary gland is provided by the following findings of a CRF-lnduced stimulation of adenylate cyclase activity in homogenate of rat and bovine pars Intermedia cells. 

The present data clearly demonstrate that the 41-amino acid ovine CRF Is a potent stimulator of adenylate cyclase activity In rat and bovine pars Intermedia tissue. Our previous data have shown that CRF causes a rapid and marked stimulation of cyclic AMP accumulation in rat pars Intermedia cells in culture (15). The final proof of the role of adenylate cyclase in the observed changes of cyclic AMP levels had to be obtained by direct measurement of adenylate cyclase activity. 

The cells in the pars intermedia of the rat pituitary produce a series of biologically active peptides, i.e., a-melanocyte-stimulating hormone, CLIP, p-endorphin, P-hpotropin, and y-MSH-related peptides. The pro-opiomelanocortin-derived peptides, especially a-MSH and P-endorphin, are suggested to be involved in learning and memory (O Dono-... 

A variety of oxidative enzymes in the pars intermedia of the pig have been demonstrated by Howe and Thody (1967). The epithelial cells of the pars intermedia of the ox showed a strong positive reaction for cytochrome oxidase, but it was not possible to differentiate between type 1 and type II cells (Raftery 1969). Sections treated with 0.05 M sodium azide as inhibitor were negative. The strong reaction for the mitochondria-associated enzymes, succinic dehydrogenase and cytochrome oxidase, indicates high oxidative activity within the cells of the intermedia. 

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