Efflux of cyclic nucleotides from rat pineal: release of guanosine 3',5'-monophosphate from sympathetic nerve endings

Cyclic GMP transporters

The biokinetics of guanosine 3',5'-cyclic monophosphate (cGMP) is characterized by three distinct processes: synthesis by guanylate cyclases (GCs), conversion of cGMP to GMP by cyclic nucleotide phosphodiesterases (PDEs) and the excretion of unchanged cGMP by transport proteins in the cell membrane. Efflux is observed in virtually all cell types including cells which originate from brain. Studies of intact cells, in which metabolic inhibitors and probenecid reduced extrusion of cGMP and wherein cGMP was extruded against concentration gradients, indicated the existence of ATP requiring organic anion transport system(s). Functional studies of inside-out vesicles have revealed cGMP transport systems wherein translocation is coupled to hydrolysis of ATP. The extrusion of cGMP is inhibited by a number of unrelated compounds and this indicates that cGMP is substrate for multispecific transporters. Recent transfection studies suggest that members of the MRP (multidrug resistance protein) family; MRP4, MRP5 and MRP8 translocate cGMP across the cell membrane. Many of the MRPs have been detected in brain. In addition tertiary active transport by the organic anion transporter family has also been identified. At least one member (OAT1) shows relative high affinity for cGMP and is also expressed in brain. The biological significance of cGMP transporters has to be clarified. Their role in cGMP biokinetics, being responsible for one of the cellular elimination pathways, is well established. However, there is growing evidence that extracellular cGMP has effects on cell physiology and pathophysiology by an auto- or paracrine mechanism.

Cyclic nucleotides and the release of vasopressin from the rat posterior pituitary gland

When rat posterior pituitary glands were stimulated by a high concentration of potassium, a peak of cyclic AMP and a peak of cyclic GMP were detected after 0.5 min and 1 min, respectively, whereas the rate of release of vasopressin was maximal only after 2 min. When calcium was omitted from the medium, no significant changes in cyclic nucleotide levels were found and the vasopressin release remained at the basal rate. During cold-stimulated (10 degrees C) release of vasopressin, a peak of cyclic AMP was detected after 5 min simultaneously with the maximal rate of vasopressin release. The significance of the cyclic nucleotides in the release of vasopressin is discussed.

Seesaw signal processing in pineal cells: homologous sensitization of adrenergic stimulation of cyclic GMP accompanies homologous desensitization of beta-adrenergic stimulation of cyclic AMP

Studies of the adrenergic regulation of cyclic GMP in the pineal gland show that (-)-norepinephrine stimulates cyclic GMP primarily in pineal cells, rather than in nerve endings as previously thought. The response exhibits the interesting and unusual characteristic of homologous sensitization: It is maintained by neural stimulation and disappears gradually as a consequence of depressed neural stimulation, due to denervation or decentralization of the superior cervical ganglia or to constant light. The response is restored in intact animals that had been in a constant-light environment when they are returned to a normal light cycle and in ganglionectomized animals by norepinephrine treatment. These findings are especially interesting because the pineal adrenergic--cyclic AMP stimulus--response system exhibits homologous desensitization. The occurrence of homologous sensitization of a cyclic GMP response and desensitization of a cyclic AMP response, which we term seesaw signal processing, in the same tissue or cell has intriguing implications. It provides a mechanism through which the qualitative nature of a multicomponent response can be modified. Such a mechanism could play a role in signal processing by neural or neuroendocrine tissues that release two or more extracellular messages.

Electric currents, bone remodeling, and orthodontic tooth movement. I. The effect of electric currents on periodontal cyclic nucleotides

Osteogenesis has been found to occur in response to the application of electric currents to bone. The objective of this experiment was to study the effects of D.C. electric currents on periodontal tissues in cats. Cyclic nucleotides, compounds known to be involved in cellular activation, were studied by immunohistochemistry in the involved tissues. Three groups of three young adult cats each were treated for 1, 3, and 7 days, respectively, by a device delivering 15 microamperes of direct current to bone osteoblasts and PDL cells stained intensely for cAMP and cGMP were observed adjacent to the cathode and anode, and bone apposition was found near the cathode. These results suggest that electric stimulation enhances cellular enzymatic phosphorylation activities in periodontal tissues and may be a potent tool in accelerating alveolar bone turnover.

Immuno-histochemical localization of cyclic nucleotides in the periodontium: mechanically-stressed cells in vivo

Cyclic AMP and cyclic GMP, considered to be cell proliferation regulators, have been reported to fluctuate in proliferating fibroblasts in vitro. The objectives of this experiment were to study the localization, distribution and staining patterns of these cyclic nucleotides in mechanically-stressed, proliferating periodontal fibroblasts in vivo. Cat canines were tipped by force applications for 0 to 48 hours and serial sagittal sections of fresh frozen, unfixed, undecalcified jaws were processed immuno-histochemically for the localization of cAMP and cGMP. Periodontal tension sites were studied microscopically. Fibroblastic staining for cAMP, which was localized mainly in the cell periphery, did not change appreciably as a result of tension. However, staining intensity increased one hour after the application of force, decreased after 6 hours and increased again at 24 hours. Staining for cGMP, initially covering the entire cell area, was concentrated over nuclei three hours after onset of tension, and diffused over cell periphery and cytoplasm thereafter. Intensity of staining for cGMP was maximal at 3 hours and low at 12 and 48 hours. These results demonstrate that stress-induced fibroblastic responses in vivo involve alterations in staining intensity for both cyclic nucleotides which may correspond with fluctuations of these regulators, reported to occur in vitro in various stages of the mitotic cycle. Moreover, this technique enables the identification of the involved cells in a stimulated, non-synchronized cell population.