Rat platelets served as a model to evaluate quantitatively how guanylate cyclase (GC)-coupled nitric oxide (NO) receptors and phosphodiesterases (here phosphodiesterase-5) interact to transduce NO signals in cells. The platelets expressed mRNA only for the ␣ 1 and ␤ 1 GC-coupled receptor subunits. In intact platelets, the potency of NO for elevating cGMP (EC 50 ‫؍‬ 10 nM) was lower than in lysed platelets (EC 50 ‫؍‬ 1.7 nM). The limiting activities of GC and phosphodiesterase in intact platelets were both very high, being equivalent to about 100 M/s. With low phosphodiesterase activity (imposed by 100 M sildenafil), the cGMP response over time was hyperbolic in shape for a range of NO concentrations or GC activities due to GC desensitization. Without a phosphodiesterase inhibitor, NO generated only brief cGMP transients, peaking after 2-5 s but amounting maximally to about 150 M cGMP. The transients were caused partly by GC desensitization, which varied in rate (halftime up to 3 s) and extent (up to 80%) depending on the NO concentration, and partly by an enhancement of the phosphodiesterase catalytic activity with time, which was deduced to be up to 30-fold and to occur with a half-time of up to 5 s. The results were simulated by a quantitative model, which also explains the varied shapes of cGMP responses to NO found in other cells. Downstream phosphorylation in platelets was detectable within 2 s, and, with continuous exposure (1 min), this pathway could be engaged by subnanomolar NO concentrations (EC 50 ‫؍‬ 0.5 nM).Whereas the spatio-temporal dynamics of some of the primary steps of cellular signal transduction, such as synaptic transmission and changes in intracellular Ca 2ϩ , have become well understood in quantitative terms (1, 2), knowledge of downstream signaling cascades remains largely qualitative. Nevertheless, the kinetic properties of the second messenger cascades are likely to be similarly instrumental in determining how cells respond.A case in point is nitric oxide (NO) 1 signaling. NO acts as a diffusible chemical messenger throughout the body, where it subserves diverse functions, including smooth muscle relaxation, inhibition of platelet aggregation, and the induction of synaptic plasticity (3,4). NO elicits these and other physiological effects by binding to its guanylyl cyclase (GC)-coupled receptors, thereby evoking the accumulation of cGMP in target cells. The receptors are ␣␤-heterodimers, of which two main isoforms, ␣ 1 ␤ 1 and ␣ 2 ␤ 1 , exist at differing levels in different tissues (5). Studies on purified receptors (6) and intact cells (7) suggest that activation of GC occurs very rapidly, within a few ms or less of adding NO, and complete deactivation in cells (upon removal of NO) requires only about 500 ms (7). Furthermore, when studied in isolation, the receptors are highly sensitive NO detectors, with only about 1 nM being needed to evoke half-maximal GC activity (8). The shape of the resulting cGMP response in cells, however, varies greatly from one cell to another. For example,...

Background and purpose: Nitric oxide (NO) acts on receptors coupled to guanylyl cyclase (GC), leading to cGMP accumulation. The NO binding site is a haem group, oxidation or loss of which diminishes NO-stimulated activity. Agonists reportedly engaging both these NO-insensitive forms have emerged. Here we characterize the effect of a prototype compound (BAY 58-2667) and use it to assess the haem status of cellular GC. Experimental approach: GC activity measurements were made on the purified protein and on rat platelets. Key results: Experiments on purified GC showed that the target for BAY 58-2667 is the haem-free GC, not the haem-oxidized form. The efficacy of BAY 58-2667 was about half that shown normally by NO. In platelets, BAY 58-2667 was a potent GC activator (EC 50 B15 nM) but the maximum effect was only about 1% of that achievable with NO. Nevertheless, it was enough to evoke cGMP-dependent protein phosphorylation. Profound (85 %) desensitization of NO-evoked GC activity did not alter the effectiveness of BAY 58-2667. Haem oxidation, however, increased the efficacy of BAY 58-2667 by 22-fold, implying that about half the cellular GC was then haem-free. Oxidation appeared to enhance the rate of haem dissociation from purified GC. Conclusions and implications:Compounds such as BAY 58-2667 are useful for probing the occupancy of the haem pocket of NO receptors in cells but not for distinguishing oxidized from reduced haem. In vivo, such compounds are likely to be particularly effective in conditions where there is deficient haem incorporation or enhanced haem loss.

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