The role of intracellular Ca2+ pool in sodium nitroprusside-induced relaxation of rat aorta smooth muscle cells

Abstract

A relaxation effect of sodium nitroprusside on smooth muscle cells of rat aorta due to intracellular Ca2+-store refilling is demonstrated using the double sucrose gap technique. It is suggested that sodium nitroprusside-induced repolarization of the smooth muscle cell membrane is associated with inhibition of Ca permeability and/or Ca-dependent K § permeability of the plasma membrane. Key words: sodium nitroprusside; Ca ions; smooth muscle cellsNitrate derivatives, in particular sodium nitroprusside (SN), activate cytosolic guanylate cyclase, which leads to a rise of intracellular cyclic GMP and triggers a cascade of cGMP-dependent events [4,9,10]. Activation of single potassium channels in smooth muscle cells (SMC) in the presence of cGMP and cGMPdependent protein kinases is extensively discussed in the literature [9,15]. Some authors suggest that this mechanism underlies the nitrate-induced relaxation of vascular SMC [3,7,15], although this is in controversy [8-121. It has been shown that cGMP inhibits both voltageoperated [8,14] and receptor-operated Ca 2 § channels [7]. The possible effect of cGMP on Ca 2 § utilization from the cytosol [2,6,13] and on the activity of contractile proteins in SMC [10,11] is intensively discussed. At the same time, simultaneous effects of nitrates on Ca 2 § influx through membrane ion channels and CaZ+release from SMC stores are disputable.The aim of the present work was to study membrane and intracellular mechanisms by which cyclic MATERIALS AND METHODSThe membrane potential and muscle tension in SMC were simultaneously measured using the double sucrose gap method [1]. Electrical potentials were recorded via nonpolarizable electrodes using a C1-83 oscilloscope coupled to a KSP-4 recorder. Contractile activity was recorded on a 6MKhlB mechanotron in a nearly isometric regimen.Male random-bred albino rats (200-250 g) were sacrificed and endothelium-denuded smooth muscle strips (0.6-0.7 mm wide, 10-12 mm long) were isolated from the middle portion of the thoracic aorta.Isolated smooth muscle strips were kept in Krebs solution (36.8~ pH 7.35) containing (in mM): 120.4 NaC1, 5.6 KCI, 15.5 NaHCO3. 1.2 MgCI 2, 1.2 NaH2PO 4, 2.5 CaC12, and 11.5 glucose for 40-45 min, and then treated with nigh-potassium (40 mM KC1) Krebs solution (Fig. 1). Test solutions were prepared on the basis of Krebs saline containing verapamil, SN (Serva), and caffeine; in the Ca-free solutions CaCI: was replaced with EGTA.

Effect of nitro derivatives on electromechanical coupling in ureteral smooth muscle cells

“…It is known that NG and SN can participate in cGMP-dependent processes acting as exogenous NO donors, activators of soluble guanylate cyclase [2][3][4][5]7]. In our experiments, methylene blue (10/aM) added to bathing solution had no effect on the parameters of AP and ureteral SMC contraction (Fig.…”

“…In contrast to the vessels [2,3,7,12] the increase in intracellular cGMP in these smooth muscles produces no relaxation. It seems that the effect of NOderivatives is tissue-specific and depends on peculiarity of excitation-contraction coupling, which does not exclude the possibility of mediation of SMC myogenic effects via cGMP-independent pathway.…”

“…It was previously reported that sodium nitroprusside (SN) induces relaxation of precontracted smooth muscle strips and repolarization of smooth muscle cell (SMC) membrane accompanied [2,3]. The effects of SN are assumed to depend on activity of soluble guanylate cyclase, because the increase in cGMP level [8] modulates calcium and potassium permeability in SMC membrane [2,3,9,14].…”

“…The effects of SN are assumed to depend on activity of soluble guanylate cyclase, because the increase in cGMP level [8] modulates calcium and potassium permeability in SMC membrane [2,3,9,14]. However, cGMP-independent pathways of relaxation induced by NO-derivatives cannot be excluded [5,15].…”

“…The method and mode of recording of SMC membrane potential and contraction were described previously [2,3].…”