Kinetic regularities of thiacalix[4]arene C-1193 action on Na+, K+-ATPase activity of the plasma membrane and contractile activity of the myometrium

Background. Nа+, K+-ATPase (sodium pump) is an electrogenic Са2+-independent Mg2+, Na+, K+-ATP-dependent transporting system of plasma membrane (PM), which conducts active transfer of univalent ions of Na and K and thus maintains their electrochemical gradients, required for normal functioning of the cell. It was proven that in some widesperad pathologies the activity of the sodium pump is disrupted. Therefore, the search for effectors - selective inhibitors and activators that would be able to specifically affect Na+, K+-ATPase, restoring its activity in pathological conditions, is very promising. The aim of this study was to investigate the action of thiacalix[4]arene С-1193 on the dependence of Nа+, K+-ATPase activity of PM of myometrium cells on the concentration of ATP and Mg ions, as well as on the isotonic contractile activity of the myometrium. Materials and Methods. The experiments were conducted using outbred white non-pregnant rats. The inhibitory action of thiacalix[4]arene C-1193 (25,27-dibutoxythiacalix[4]arene-bis-hydroxymethylphosphonic acid) on the kinetic traits of Na+, K+-ATPase activity was studied in the experiments, conducted using the suspension of perforated plasma membranes of the myometrium cells. The registration of the contractile acti­vity in the preparations of longitudinal smooth muscles of uterine horns with preserved endothelium was done in the isotonic mode. The study of the contractile activity of muscle preparations was carried out using mechanokinetic analysis methods. Results. The study demonstrated that while inhibiting Na+, K+-ATPase, thiacalix[4]arene С-1193 did not change the kinetic parameters (Km, nH) of the dependence of the reaction velocity on the substrate concentration. Calix[4]arene C-1193 had practically no action on the affinity between Na+, K+-ATPase and ATP, which demonstrated the absence of competition between the binding centers for ATP and C-1193. There was no effect on the affinity and cooperative action of Mg ions either. Both cases demonstrated a considerable reduction in the maximal velocity of ATP hydrolysis. It was found that thiacalix[4]arene С-1193 (in the concentration of 10 μM) modulated the isotonic reactions of pluricellular preparations of myometrium, induced via the pathways of electro- and pharmacomechanic coupling. It was also determined that under the effect of С-1193, there was an increase both in the amplitude of contractions and in the mechanokinetic parameters: contractions (ΔLmax, ΔLC and ΔLR) and velocities (VC and VR). The norm-setting for VC and VR regarding the amplitude of contractions at the action of С-1193 resulted in the loss of statistically significant differences between the maximal velocities of the contraction and relaxation phases. Conclusions. The results of our research indicate that thiacalix[4]arene С-1193, has a non-competitive mechanism of inhibiting Nа+, K+-ATPase activity and has no specific action on Са2+-transporting systems of uterine myocytes.