EFFECTS OF VASOACTIVE AGONISTS ON THE MEMBRANE-POTENTIAL OF CULTURED BOVINE AORTIC AND GUINEA-PIG CORONARY ENDOTHELIUM

1. The effects of bradykinin, ATP, adenosine, histamine and thrombin on the membrane potential of confluent monolayers of cultured bovine aortic endothelial cells (BAECs) and guinea-pig coronary endothelial cells (GCECs) were studied at 37-degrees-C using the whole-cell mode of the patch-clamp technique. 2. The amplitude histogram of the resting potentials of BAEC monolayers showed a bimodal distribution with one peak around -25 mV and another peak around -85 mV. Transitions from one potential level to the other were observed. The bistable membrane potential can be explained by an N-shaped current-voltage relation of the endothelial cell membrane. 3. When BAECs with a low resting potential (-10 to -30 mV) were superfused with maximally effective concentrations of ATP (2-10-mu-M) an initial hyperpolarization of -80 to -90 mV was observed which decayed to a plateau of about -60 mV within 1 min. When ATP was removed after 2-3 min the membrane potential returned to control level within 1 min. This was followed by a second hyperpolarization of 10-20 mV, which decayed within 15 min. 4. In the absence of extracellular calcium, ATP produced only a brief transient hyperpolarization in aortic endothelium. The plateau and the secondary hyperpolarization were abolished. These findings are consistent with the idea that the changes in membrane potential reflect changes in intracellular free Ca2+ and that the initial peak is due to release of Ca2+ from intracellular stores, whereas the plateau and the secondary hyperpolarization depend on transmembrane Ca2+ influx. 5. Bradykinin evoked potential changes similar to ATP in BAECs, except that the secondary hyperpolarization during wash-out was absent. When the membrane potential was more negative than -80 mV, ATP and bradykinin induced only a small initial hyperpolarization followed by a depolarization of up to 20 mV. 6. In aortic endothelium, ADP (10-mu-M) evoked a much smaller response than ATP. Adenosine (10-mu-M), thrombin (2 units/ml), acetylcholine (10-mu-M) and histamine (10-mu-M) had only a very small effect on the membrane potential, if any. 7. The amplitude histogram of the membrane potential of GCECs showed only one peak around -35 mV. In coronary endothelium, application of bradykinin, ATP, histamine, thrombin, acetylcholine and adenosine all evoked a transient hyperpolarization of 10-40 mV lasting 1 min or less, which then turned into a depolarization. 8. The K+ channel openers cromakalim (BRL 34915) and lemakalim (BRL 38227) did not affect the membrane potential of GCECs or BAECs. This suggests that ATP-sensitive K+ channels are absent in endothelial cells. 9. Reducing the oxygen tension (P(O2)) of the superfusate from 150 mmHg to 12-32 mmHg also had virtually no effect on the resting potential of BAECs and GCECs. However, in sixteen out of twenty-six experiments 15 min hypoxia produced an increase in input resistance; the median of the increase was 9%. This may be due to partial uncoupling of the cells in the monolayer caused by closure of intercellular gap junctions. 10. It is concluded that in spite of many common properties there are some marked differences between coronary (mainly microvascular) and aortic (macrovascular) endothelial cells in their electrical response to vasoactive substances. The implications of the present findings for the mechanisms of hypoxic vasodilatation in the heart are discussed.