Differential effects of soluble and particulate guanylyl cyclase on Ca²⁺ sensitivity in airway smooth muscle

Maximal relaxation of airway smooth muscle (ASM) in response to atrial natriuretic peptide (ANP), which stimulates particulate guanylyl cyclase (pGC), is less than that produced by nitric oxide (NO) and other compounds that stimulate soluble guanylyl cyclase (sGC). We hypothesized that stimulation of pGC relaxes ASM only by decreasing intracellular Ca²⁺ concentration ([Ca²⁺]_i), whereas stimulation of sGC decreases both [Ca²⁺]_i and the force developed for a given [Ca²⁺]_i (i.e., the Ca²⁺ sensitivity) during muscarinic stimulation. We measured the relationship between force and [Ca²⁺]_i (using fura 2) under control conditions (using diltiazem to change [Ca²⁺]_i) and during exposure to ANP, diethylamine-NO (DEA-NO), sodium nitroprusside (SNP), and the Sp diastereoisomer of β-phenyl-1,N²-etheno-8-bromoguanosine-3′,5′ -cyclic monophosphorothionate (Sp-8-Br-PET-cGMPS), a cell-permeant analog of cGMP. Addition of DEA-NO, SNP, or Sp-8-Br-PET-cGMPS decreased both [Ca²⁺]_i and force, causing a significant rightward shift of the force-[Ca²⁺]_i relationship. In contrast, with ANP exposure, the force-[Ca²⁺]_i relationship was identical to control, such that ANP produced relaxation solely by decreasing [Ca²⁺]_i. Thus, during muscarinic stimulation, stimulation of pGC relaxes ASM exclusively by decreasing [Ca²⁺]_i, whereas stimulation of sGC decreases both [Ca²⁺]_i and Ca²⁺ sensitivity.