Calix[4]arene C-1220 - Modulator of calcium homeostasis and contractile activity in smooth muscle cells

Regulation of Na⁺,K⁺-ATPase activity plays a critical role in maintaining ionic homeostasis and Ca²⁺-dependent contractility in smooth muscle, yet the contribution of selective sodium pump modulators to myometrial function remains insufficiently understood. In this study, we investigated the effects of calix[4]arene C-1220 (25,27-dipropoxycalix[4]arene-bis-N-tolylsulfonylaminomethylphosphonic acid) and its structural analogue C-99 on ATP-hydrolyzing activities, intracellular Ca²⁺ dynamics, mitochondrial function, and contractile behavior of uterine smooth muscle. Enzymatic assays showed that C-1220 is a highly potent and selective inhibitor of Na⁺,K⁺-ATPase (IC₅₀ = 48 ± 2 nM) without influencing other ATPases in the plasma membrane. Confocal imaging demonstrated that C-1220 elevates cytosolic Ca²⁺ in myocytes, whereas flow cytometry and Ca²⁺-accumulation assays indicated that mitochondrial membrane polarization and matrix Ca²⁺ levels remain unaffected. Tensometric measurements revealed that both C-1220 and C-99 enhance spontaneous contractions and force-, velocity-, and impulse-related mechanokinetic parameters; however, normalization of velocity parameters suggests that these changes do not reflect direct modulation of Ca²⁺-transporting systems. Comparative analysis supports the conclusion that inhibition of Na⁺,K⁺-ATPase and consequent disturbance of Na⁺/Ca²⁺ exchange underlie the contractile activation induced by both compounds. These findings provide new insights into membrane mechanisms governing electro- and pharmacomechanical coupling in uterine smooth muscle and highlight C-1220 as a useful molecular tool for probing Na⁺,K⁺-ATPase-dependent Ca²⁺ regulation.