ELECTRICAL-PROPERTIES OF RESTING AND ACETYLCHOLINE-STIMULATED ENDOTHELIUM IN INTACT RAT AORTA

1. The passive electrical properties and the effects of acetylcholine on the membrane potential of the endothelium of intact rat aorta were investigated using the whole cell mode of the patch clamp technique. 2. Unstimulated endothelium had a membrane potential of - 58 +/- 8 mV (S.E.M., n = 193; range -47 to -76 mV). The input resistance was 43 +/- 13 MOMEGA (s.E.M., n = 8; range 26-64 MOMEGA). KCl and BaCl2, but not tetraethylammonium (2 mm), 4-aminopyridine (5 mm) or 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS; 100 mum) depolarized the endothelium. 3. Acetylcholine (0.2-4 mum) evoked in most preparations a biphasic response with a transient hyperpolarization to a value close to the K+ reversal potential, followed by depolarization beyond the resting potential. In 46 % of recordings, the depolarization was followed by oscillations in membrane potential. The duration of the hyperpolarization and magnitude of the depolarization was similar in all recordings from a given aorta, but varied greatly between different preparations. 4. Hyperpolarization of the endothelium below the K+ reversal potential reversed the direction of the first phase of the acetylcholine-evoked response, which was unaffected by tetraethylammonium, 4-aminopyridine, or DIDS. 5. The removal of extracellular Ca2+ evoked a depolarization of the endothelium from - 61 +/- 3 to - 34 +/- 3 mV (S.E.m., n = 9) over 2-15 min. Restoration of external Ca2+ evoked a transient hyperpolarization. 6. ACh applied in nominally Ca2+-free medium shortly after Ca2+ removal evoked only a transient hyperpolarization. After the establishment of a stable membrane potential in Ca2+-free medium, acetylcholine was without effect. 7. NiCl2 (2 mm) evoked a small depolarization of the endothelium (6 +/- 2 mV; s.E.m., n = 7). The subsequent removal of Ni2+ evoked a transient hyperpolarization. 8. In the presence of Ni2+, acetylcholine evoked a short-lived hyperpolarization. Both the application of Ni2+ and the removal of extracellular Ca2+ immediately blocked oscillations in membrane potential evoked by acetylcholine. 9. The blockers of voltage-operated Ca2+ channels, nifedipine (1-10 mum) and verapamil (20 mum) were without effect on the biphasic acetylcholine-evoked responses. 10. In preparations in which acetylcholine evoked large (20-45 mV) oscillations in membrane potential, depolarization of the endothelium alone, by current injection or application of KCl, did not evoke oscillations. 11. The activator of protein kinase C, phorbol 12, 13-dibutyrate (200 nM) depolarized and greatly increased the input resistance of the endothelium, presumably due to an effect on gap junctions. The blockers of protein kinase C, staurosporine (200 nm) and H-7 (200 nM), were without effect on acetylcholine-evoked responses. 12. These results suggest that acetylcholine evokes complex changes in endothelial membrane potential following changes in cytosolic calcium concentration. The initial agonist-evoked hyperpolarization appeared dependent on the release of Ca2+ from intracellular stores, whilst the prolonged phase of the hyperpolarization, depolarization and oscillations in membrane potential are dependent upon the entry of Ca2+ through Ni2+-sensitive channels. Protein kinase C does not appear to be involved in any phase of the acetylcholine-evoked response.