Alterations of cardiac contractile function are related to changes in membrane calcium transport in spontaneously hypertensive rats.
OBJECTIVE
Transport activities of cardiac sodium-calcium exchange and sarcoplasmic reticulum calcium ATPase were measured biochemically in spontaneously hypertensive rats (SHR) with hypertrophied myocardium and in normotensive Wistar-Kyoto (WKY) rats and it was tested whether possible differences have consequences for the contractile properties of papillary muscle.


METHODS
Sarcolemmal sodium-dependent calcium transport via sodium-calcium exchange and oxalate-supported sarcoplasmic reticulum calcium uptake were measured in left ventricular membranes of 22-week-old rats. Postextrastimulatory potentiated contractions, postrest potentiated contractions, the twitch-to-twitch decay of those potentiations and the response to increasing stimulation frequency of left ventricular papillary muscles were analysed.


RESULTS
Compared with WKY rats we found in SHR: a significant increase in sodium-calcium exchange (65%) and in sarcoplasmic reticulum calcium uptake (24%); a steeper twitch-to-twitch decay in postextrastimulatory potentiated contractions and postrest potentiated contractions, suggesting a lower calcium fraction recirculating between myofilaments and sarcoplasmic reticulum and, consequently, a relatively higher calcium efflux via sodium-calcium exchange; a stronger rest-dependent decrease in recirculating calcium fraction in postrest potentiated contractions accompanied by accelerated relaxation, suggesting an increasing driving force for calcium extrusion via sodium-calcium exchange, probably caused by decreasing intracellular sodium during rest; a greater transient decrease in peak force of subsequent twitches after postrest potentiated contractions below pre-interventional level, indicating higher cellular calcium loss; and a smaller negative inotropic effect in response to doubling of stimulation rate as a manoeuvre to increase the intracellular sodium level.


CONCLUSIONS
In SHR, the contractile properties suggest an increased contribution of sodium-calcium exchange to cellular calcium removal, which is strongly supported by the enhanced sodium-calcium exchange activity in cardiac membrane vesicles.
Lammerich
A
A
Institute of Physiology, Humboldt-University, Berlin, Germany.
Günther
J
J
Pfitzer
G
G
Storch
E
E
Vetter
R
R
D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D004558:Electric Stimulation / Q000379:methods; D006973:Hypertension / Q000503:physiopathology*; D008297:Male; D008566:Membranes / Q000378:metabolism; D009200:Myocardial Contraction; D010070:Oxalates / Q000494:pharmacology; D051381:Rats; D011918:Rats, Inbred SHR; D011921:Rats, Inbred WKY; D012964:Sodium / Q000378:metabolism
D010070:Oxalates; D012964:Sodium; D002118:Calcium