Demarcation of Ca2+ transport processes in guinea pig stomach smooth muscle

Functional differences of Na+/Ca2+ exchanger expression in Ca2+ transport system of smooth muscle of guinea pig stomach

The plasma membrane ATP-dependent Ca2+ pump and the Na+/Ca2+ exchanger (NCX) are the major means of Ca2+ extrusion in smooth muscle. However, little is known regarding distribution and function of the NCX in guinea pig gastric smooth muscle. The expression pattern and distribution of NCX isoforms suggest a role as a regulator of Ca2+ transport in cells. Na+ pump inhibition and the consequent to removal of K+ caused gradual contraction in fundus. In contrast, the response was significantly less in antrum. Western blotting analysis revealed that NCX1 and NCX2 are the predominant NCX isoforms expressed in stomach, the former was expressed strongly in antrum, whereas the latter displayed greater expression in fundus. Isolated plasma membrane fractions derived from gastric fundus smooth muscle were also investigated to clarify the relationship between NCX protein expression and function. Na+-dependent Ca2+ uptake increased directly with Ca2+ concentration. Ca2+ uptake in Na+-loaded vesicles was markedly elevated in comparison with K+-loaded vesicles. Additionally, Ca2+ uptake by the Na+- or K+-loaded vesicles was substantially higher in the presence of A23187 than in its absence. The result can be explained based on the assumption that Na+ gradients facilitate downhill movement of Ca2+. Na+-dependent Ca2+ uptake was abolished by the monovalent cationic ionophore, monensin. NaCl enhanced Ca2+ efflux from vesicles, and this efflux was significantly inhibited by gramicidin. Results documented evidence that NCX2 isoform functionally contributes to Ca2+ extrusion and maintenance of contraction-relaxation cycle in gastric fundus smooth muscle.

Pump and exchanger mechanisms in a model of smooth muscle

A novel approach to modelling pump and exchanger mechanisms is presented. In this approach, new thermodynamic expressions for the calcium pump, sodium-calcium exchanger and sodium-potassium pump are developed using statistical rate theory (SRT). This theory is well-defined and is not derived empirically. This is in contrast to previous thermodynamic pump expressions which used a simple linear relationship or relied on empirical data for their functional form. The functional form of these new expressions does not require assumptions of steady state or particular forms of voltage dependencies in specific steps. Also, the explicit reaction scheme is not required. Instead, assumptions of a rate-limiting step in the scheme and a near-equilibrium ratio of intermediate substrates are required. These expressions are incorporated into an overall model of gastric smooth muscle. This model presents a novel approach whereby thermodynamic representations for calcium pumps, sodium-calcium exchangers and sodium-potassium pumps have been included together in a model of ionic transport mechanisms for smooth muscle. Variations in basal metabolic concentrations are used to explain the observed amplitude variation in the transmembrane voltage of gastric smooth muscle. The interaction of the various mechanisms are used to illustrate the large depolarization obtained in smooth muscle with ouabain as well as the forward and reverse modes of the sodium-calcium exchanger.