cGMP abolishes agonist-induced [Ca(2+)](i) oscillations in human bladder epithelial cells

A potentiating effect of endogenous NO in the physiologic secretion from airway submucosal glands

It is known that several second messengers, such as Ca(2+) or cAMP, play important roles in the intracellular pathway of electrolyte secretion in tracheal submucosal gland. However, the participation of cGMP, and therefore nitric oxide (NO), is not well understood. To investigate the physiologic role of NO, we first examined whether tracheal glands can synthesize NO in response to acetylcholine (ACh), and then whether endogenous NO has some effects on the ACh-triggered ionic currents. From the experiments using the NO-specific fluorescent indicator 4,5-diaminofluorescein diacetate salt (DAF-2DA), we found that a physiologically relevant low dose of ACh (100 nM) stimulated the endogenous NO synthesis, and it was almost completely suppressed in the presence of the nonspecific NO synthase (NOS) inhibitor Nomega-Nitro-L-arginine Methyl Ester Hydrochloride (L-NAME) or the neuronal NOS (nNOS)-specific inhibitor 7-Nitroindazole (7-NI). Patch-clamp experiments revealed that both the NOS inhibitors (L-NAME or 7-NI) and cGK inhibitors (KT-5823 or Rp-8-Br-cGMP) partially decreased ionic currents induced by 30 nM of ACh, but not in the case of 300 nM of ACh. Our results indicate that NO can be synthesized through the activation of nNOS endogenously and has potentiating effects on the gland secretion, under a physiologically relevant ACh stimulation. When cells were stimulated by an inadequately potent dose of ACh, which caused an excess elevation in [Ca(2+)](i), the cells were desensitized. Therefore, due to NO, gland cells become more sensitive to calcium signaling and are able to maintain electrolyte secretion without desensitization.

Reciprocal regulation and integration of signaling by intracellular calcium and cyclic GMP

Calcium and guanosine-3',5'-cyclic monophosphate (cGMP) are second messenger molecules that regulate opposing physiological functions, reflected in the reciprocal regulation of their intracellular concentrations, in many systems. Indeed, cGMP and Ca2+ constitute discrete points of integration between multiple cell signaling cascades in both convergent and parallel pathways. This chapter describes the molecular mechanisms regulating intracellular Ca2+ and cGMP, and their integration in specific cellular responses.

The role of cGMP in the regulation of rabbit airway ciliary beat frequency

The involvement of cyclic guanosine 3',5'-monophosphate (cGMP) and cGMP-dependent protein kinase (PKG) and their interaction with the Ca2+-dependent mechanisms in the regulation of ciliary activity are not well understood. To investigate how cGMP regulates ciliary activity, changes in ciliary beat frequency (CBF) and intracellular calcium concentration ([Ca2+]i) of rabbit tracheal ciliated cells in response to 8-bromo-cGMP (Br-cGMP) were simultaneously quantified using digital, high-speed phase-contrast and fluorescence imaging. Br-cGMP induced a response in ciliary activity that could be separated into two parts. Firstly, Br-cGMP induced a concentration-dependent increase in the basal CBF that occurred without increasing the [Ca2+]i. This response was not affected by excessively buffering the [Ca2+]i with BAPTA but was abolished by KT5823, a PKG inhibitor. Secondly, Br-cGMP induced a series of transient increases in CBF that were superimposed on the sustained increases in CBF. These transient increases in CBF correlated with the stimulation of a series of transient increases in [Ca2+]i and were abolished by BAPTA, but were unaffected by KT5823. The magnitude of the transient increases in CBF and [Ca2+]i were not dependent on the concentration of Br-cGMP. The Ca2+-dependent changes in CBF induced by ionomycin or ATP were not affected by KT5823. From these results, we propose that cGMP increases CBF in two ways: firstly through a Ca2+-independent mechanism involving PKG, and secondly through a Ca2+-dependent mechanism following the stimulation of changes in [Ca2+]i. In addition, we suggest that the Ca2+-dependent stimulation of rabbit airway ciliary activity does not initially require PKG activation.