Pathway-specific Effects Research Articles

Pathway-specific effects of calcitonin gene-related peptide on irideal arterioles of the rat.

1. Arteriolar diameter and membrane voltage have been measured to investigate the actions of calcitonin gene-related peptide (CGRP) in rat irideal arterioles. 2. Activation of sensory nerves inhibited sympathetic vasoconstriction, reduced the accompanying 40-50 mV depolarization by 90% and caused a 4 mV hyperpolarization. 3. The inhibition of vasoconstriction was prevented by either preincubation in L-NAME (10 microM), to inhibit nitric oxide production, by preincubation in the cell-permeant adenylate cyclase inhibitor dideoxyadenosine (1 mM) or by preincubation in the ATP-sensitive potassium channel blocker glibenclamide (10 microM). The subsequent addition of a nitric oxide donor to the glibenclamide solution inhibited nerve-mediated vasoconstriction, suggesting that the potassium channel involvement preceded the production of nitric oxide. The small hyperpolarization was not affected by L-NAME. 4. Nerve-mediated vasodilatation persisted in the presence of L-NAME (10 microM) but was abolished with the CGRP1 receptor antagonist CGRPS-37. 5. In arterioles preconstricted with the alpha 2-adrenoceptor agonist UK-14304 (100 nM), exogenous CGRP caused a hyperpolarization and a dose-dependent vasodilatation, neither of which was affected by L-NAME (10 microM). 6. In arterioles preconstricted with 30 mM KCl, CGRP (10 nM) caused vasodilatation but not hyperpolarization, suggesting that the hyperpolarization was not causal to the vasodilatation. 7. Forskolin (30 nM), in the presence of L-NAME to present effects due to nitric oxide, caused vasodilatation. 8. These results suggest that CGRP inhibits sympathetic nerve-mediated vasoconstriction through sequential increases in cyclic AMP and nitric oxide, while vasodilatation results from increases in cyclic AMP alone. The production of nitric oxide, but not its mechanism of action, appears to be dependent on the activation of ATP-sensitive potassium channels. The possible sites of action of these two pathways are discussed.

Combinatorial control of cell differentiation by cAMP and DIF-1 during development of Dictyostelium discoideum

At least three distinct types of cell arise from a population of similar amoebae during Dictyostelium development: prespore, prestalk A and prestalk B cells. We report evidence suggesting that this cellular diversification can be brought about by the combinatorial action of two diffusible signals, cAMP and DIF-1. Cells at different stages of normal development were transferred to shaken suspension, challenged with various combinations of signal molecules and the expression of cell-type-specific mRNA markers measured 1-2 h later. pDd63, pDd56 and D19 mRNAs were used for prestalk A, prestalk B and prespore cells respectively. We find the following results. (1) Cells first become responsive to DIF-1 for prestalk A differentiation and to cAMP for prespore differentiation at the end of aggregation, about 2 h before these cell types normally appear. (2) At the first finger stage of development, when the rate of accumulation of the markers is maximal, the expression of each is favoured by a unique combination of effectors: prespore differentiation is stimulated by cAMP and inhibited by DIF-1; prestalk A differentiation is stimulated by both cAMP and DIF-1 and prestalk B differentiation is stimulated by DIF-1 and inhibited by cAMP. (3) Half-maximal effects are produced by 10-70 nM DIF-1, which is in the physiological range. (4) Ammonia and adenosine, which can affect cell differentiation in other circumstances, have no significant pathway-specific effect in our conditions. These results suggest that cell differentiation could be brought about in normal development by the localized action of cAMP and DIF-1.