Aminophylline Pretreatment Modulates the Nitric Oxide System of the Isolated Rat Diaphragm: the Role of Frequency of the Electrical Stimulation

Abstract

Abstract. The effect of different frequencies of direct subtetanic electrical stimulation (14, 20, and 30 Hz) and aminophylline (AMPh) pretreatment on the effect of N G -nitro-L-arginine methyl ester (L-NAME) (1 mM) on isolated rat diaphragm was investigated. L-NAME potentiated tension developed (Td) in the diaphragm pretreated with a single concentration of AMPh (1.08 mM) in a frequency-dependent manner. The effect was significantly different in comparison with muscle incubated 30 min with Tyrode solution only. In the muscle pretreated with cumulative concentrations of AMPh (0.36 -3.60 mM), the frequency-dependent potentiation of Td induced by L-NAME and the difference between L-NAME-treated and untreated muscle were lost.After the discovery of the nitric oxide (NO) system in skeletal muscle, the influence of NO and inhibitors of NO synthase (NOS) on muscle contractility were investigated (1, 2). Kobzik et al.(1) have shown that blockade of NOS in the isolated rat diaphragm produced an increase of muscle contractility. They postulated that inhibitors of NO (i.e., 7-nitroindazole, aminoguanidine, and nitro-L-arginine) have a specific, NO-related effect on the muscle contractility. However, some authors observed NO-unrelated effects of NOS inhibitors (3).NO is synthesized continually at low rates by constitutive isoforms of NOS, the synthesis of NO being increased during repetitive isometric contractions up to 200% (4). NO may modulate skeletal muscle contractility via at least two mechanisms, through the cGMP-NO pathway, and directly, by interacting with calcium release channels of the sarcoplasmic reticulum (5). The effects of NO on the channel activity primarily depend on its concentration, and its interaction with reactive oxygen species (ROS) (2). Reid et al. (6) have shown that superoxide generated by diaphragm muscle bundles increases both twitch and tetanic forces developed, a deesterifed form of L-NAME, in the concentration range used in our experiments (≥1 mM) blocked NOS activity in the skeletal muscle (e.g., rat diaphragm, extensor digitorum longus) (1,7,8).The diaphragm is the primary muscle of inspiration. The relationship between the diaphragm contractile function and NO-synthase activity could have therapeutic significance in neuromuscular diseases associated with changes in muscle fibers and activity of the NO system. Consequently, the effects of various NOS inhibitors on diaphragm contractility have been evaluated during the last decade (1, 2).The interaction between L-NAME and aminophylline (AMPh) on the contractility of the isolated rat diaphragm stimulated by direct subtetanic electrical stimulation (DSES) and the role of extracellular calcium in such an interaction were already investigated. It was shown that the stimulatory effects of cumulative concentrations of AMPh on the contractility of the isolated rat diaphragm during DSES (14 Hz) were potentiated by L-NAME. In a calcium-free medium, L-NAME completely abolished the effect of AMPh on diaphragm

Rat skeletal muscle contractility: The role of beta-adrenoceptors and nitric oxide system

“…In previous studies, propranolol (1 µmol/L; 30 min incubation) did not change Td itself, but abolished the stimulatory effect of L-NAME in the rat hemidiaphragm (Stojanovi} et al, 2005). On the other hand, atenolol (1 mmol/L, 30 min incubation) neither significantly changed Td nor L-NAME-induced potentiation of Td.…”

“…In the stimulatory action of AMPh on muscle contractility are probably involved both the enhanced calcium release from SR and the increased influx of extracellular calcium through sarcolemmal L-calcium channels (Prostran et al, 1993;Stojanovi} et al, 2004Stojanovi} et al, , 2005.…”

Type of Electrical Stimulation Influences Diaphragm Response to Adrenoceptor and Calcium Channel Modulators: the Role of Extracellular and Intracellular Calcium Events

“…The experiments were performed on the isolated rat hemidiaphragm (1,12,13). The muscle was immersed in a Tyrode solution (T = 36°C) with a double amount of glucose and bubbled with a mixture of 97% O 2 and 3% CO 2 .…”