EFFECT OF ENDOTHELIN-1 ON NA+-K+ PUMP CURRENT IN CARDIAC MYOCYTES
C. Fernandes*, N. Bewick, H.H. Rasmussen.
Cardiology Department, Royal North Shore Hospital, St Leonards, New South Wales.
Endothelin is emerging as an important cardiovascular hormone. Since it activates tyrosine kinase and since tyrosine kinase activation has been reported to enhance the Na+-K+ pump's sensitivity to intracellular Na+, we examined if endothelin regulates the pump. Rabbit ventricular myocytes were voltage clamped at -40mV. The Na+ concentration in pipette filling solutions (Napip) was at a near physiological intracellular level (10mM) or at a level expected to saturate intracellular pump sites (80mM). Na+-K+ pump current (Ip) was identified as the shift in holding current induced by 10-4M ouabain. Ip was measured in controls and in myocytes exposed for -5min to 10-8M endothelin-1 (ET-1) after the whole-cell configuration had been established. When Napip was 10mM mean Ip (±SE) of 13 myocytes exposed to ET-1 (0.49±0.02 pA/pF) was significantly (P<0.01) larger than mean Ip of 8 controls (0.36±0.02 pA/pF). When Napip was 80mM mean Ip of 10 myocytes exposed to ET-1 (1.99±0.04 pA/pF) and mean Ip of 11 myocytes of controls (1.94±0.09 pA/pF) were similar. Since the ET-1A receptor is the predominant ET-1 receptor expressed in cardiac myocytes, we exposed 5 myocytes to both ET-1 and 10-6M of the specific antagonist BQ-123 (Napip 10mM). Mean Ip (0.32±0.02 pA/pF) was significantly smaller than mean Ip of the myocytes exposed to ET-1 only (P<0.01). To examine if tyrosine kinase might be involved in the effect of ET-1 on Ip, we included 10-4 M of the tyrosine kinase inhibitor tyrphostin A25 (Tyr-A25) in pipette filling solutions. Mean Ip of 16 myocytes exposed to ET-1 and Tyr-A25 (0.31±0.02 pA/pF) was significantly (P<0.01) less than mean Ip of myocytes exposed to ET-1 alone. We conclude that brief exposure of cardiac myocytes to ET-1 stimulates Ip when Napip is near physiological intracellular levels. This involves the ET-1A receptor and tyrosine kinase.
