Characterization of Ca2+ transport and enzyme activity in microsomes isolated from guinea-pig stomach smooth muscle

Ca2+ uptake by microsomes prepared from guinea-pig stomach required the presence of both ATP and Mg2+ and was unaffected by NaN3. ATP-dependent Ca2+ uptake increased with increasing free Ca2+ concentration from 0.1 to 5 microM, and further increase in Ca2+ concentration above 5 microM did not enhance the uptake further. Half-saturation occurred at approximately 0.55 microM. The t1/2 values of Ca2+ loss from these vesicles loaded in the presence of oxalate were significantly slower than those in the absence of oxalate. Enzyme activity suggested linkage between Ca2+ uptake and ATPase activity, and most of the azide-sensitive component of ATP hydrolysis was attributable to potent inhibition of ADPase activity.