title: Role of phosphatidylinositol in cardiac sarcolemmal membrane sodium-calcium exchange.
abstract: The purpose of this investigation was to study the effects of a distinct type of phospholipase C on sarcolemmal Na+-Ca2+ exchange. With this phospholipase C (Staphylococcus aureus), treatment of cardiac sarcolemmal vesicles resulted in a specific hydrolysis of membrane phosphatidylinositol. This hydrolysis of phosphatidylinositol also released two proteins (110 and 36 kDa) from the sarcolemmal membrane. Phospholipase C pretreatment of the sarcolemma resulted in an unexpected stimulation of Na+-Ca2+ exchange. The Vmax of Na+-Ca2+ exchange was increased but the Km for Ca2+ was not altered. This stimulation was specific to the Na+-Ca2+ exchange pathway. ATP-dependent Ca2+ uptake was depressed after phospholipase C treatment, but passive membrane permeability to Ca2+ was unaffected. Sarcolemmal Na+,K+-ATPase activity was not altered, whereas passive Ca2+ binding was modestly decreased after phospholipase C pretreatment. The stimulation of Na+-Ca2+ exchange after phosphatidylinositol hydrolysis was greater in inside-out vesicles than in a total population of vesicles of mixed orientation. This finding suggests that the cardiac sarcolemmal Na+-Ca2+ exchanger is functionally asymmetrical. The results also suggest that membrane phosphatidylinositol is inhibitory to the Na+-Ca2+ exchanger or, alternatively, this phospholipid may anchor an endogenous inhibitory protein in the sarcolemmal membrane. The observation that a transsarcolemmal Ca2+ flux pathway may be stimulated solely by phosphatidylinositol hydrolysis independently of phosphoinositide metabolic products like inositol triphosphate is novel.
authors.affiliation: Division of Cardiovascular Sciences, St. Boniface General Hospital Research Centre, Winnipeg, Canada.
authors.forename: G N
authors.initials: GN
authors.lastname: Pierce
authors.forename: V
authors.initials: V
authors.lastname: Panagia
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism*; D004285:Dogs; D006352:Heart Ventricles / Q000378:metabolism; D007700:Kinetics; D008563:Membrane Lipids / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism*; D010716:Phosphatidylinositols / Q000378:metabolism*; D010743:Phospholipids / Q000378:metabolism; D012508:Sarcolemma / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D013211:Staphylococcus aureus / Q000201:enzymology; D010738:Type C Phospholipases / Q000494:pharmacology
chemical_list: D002352:Carrier Proteins; D008563:Membrane Lipids; D010716:Phosphatidylinositols; D010743:Phospholipids; D019831:Sodium-Calcium Exchanger; D010738:Type C Phospholipases; D000252:Calcium-Transporting ATPases; D000254:Sodium-Potassium-Exchanging ATPase; D002118:Calcium