The hypoxic constriction of isolated pulmonary vessels is composed of an initial transient phase (phase 1) followed by a slowly developing increase in tone (phase 2). We investigated the roles of the endothelium and of intracellular Ca2+ stores in both preconstricted and unpreconstricted intrapulmonary rabbit arteries when challenged with hypoxia (PO2 16-21 Torr). Removing the endothelium did not affect phase 1, but phase 2 appeared as a steady plateau. Removing extracellular Ca2+ had essentially the same effect as removing the endothelium. Depletion of sarcoplasmic reticulum Ca2+ stores with caffeine and ryanodine abolished the hypoxic response. Omitting preconstriction reduced the amplitude of the hypoxic response but did not qualitatively affect any of the above responses. We conclude that hypoxia releases intracellular Ca2+ from ryanodine-sensitive stores by a mechanism intrinsic to pulmonary vascular smooth muscle without the need for Ca2+ influx across the plasmalemma or an endothelial factor. Our results also suggest that extracellular Ca2+ is required for the release of an endothelium-derived vasoconstrictor.

SUMMARY1. The dual-emission pH-sensitive fluoroprobe carboxy-SNARF-1 (carboxyseminaptharhodofluor) was used to measure pHi in type I cells enzymically dispersed from the neonatal rat carotid body.2. Steady-state pHi in cells bathed in a HEPES-buffered Tyrode solution (pH 7 4) was found to be remarkably alkaline (pHi = 7-77) whereas cells bathed in a C02-HC03--buffered Tyrode solution (pH 74) had a more 'normal' pHi (pHi = 7-28). These observations were further substantiated by using an independent nullpoint test method to determine pHi.3. Intracellular intrinsic buffering (,f, determined by acidifying the cell using an NH4Cl pre-pulse) was in the range 7-20 mm per pH unit and appeared to be dependent upon pHi with /8 increasing as pHi decreased.4. In cells bathed in a HEPES-buffered Tyrode solution, pHi recovery from an induced intracellular acid load (10 mM-NH4Cl pre-pulse) was inhibited by the Na+-H+ exchange inhibitor ethyl isopropyl amiloride (EIPA; 150 1uM) or substitution of Na+ with N-methyl-D-glucamine (NMG). Both EIPA and Na+ removal also caused a rapid intracellular acidification, which in the case of Na+ removal, was readily reversible. The rate of this acidification was similar for both Na+ removal and EIPA addition. 5. Transfering cells from a HEPES-buffered Tyrode solution to one buffered with 5% C02-HCO3-resulted in an intracellular acidification which was partially, or wholly, sustained. The rate of acidification upon transfer to C02-HCO3-was considerably slowed by the membrane permeant carbonic anhydrase inhibitor, acetazolamide, thus indicating the presence of the enzyme in these cells.6. In C02-HC03--buffered Tyrode solution, pHi recovery from an intracellular acidosis (NH+ pre-pulse) was only partially inhibited by EIPA or amiloride whereas Na+ removal completely inhibited the recovery. The stilbene DIDS (4,4-diiso- K. J. BUCKLER AND OTHERS 150 ,tM-EIPA. We conclude, that in the presence of C02-HCO3-, a Na+-and HCO3--dependent (DIDS inhibitable) mechanism aids acid extrusion.7. The application of DIDS to cells in C02-HC03--buffered Tyrode solution was frequently observed to cause a small alkalosis ( t 0 1 pH units) suggesting that DIDS may also inhibit a mechanism which mediates a background acid influx in the steady state.8. Removal of external Cl-(glucuronate and gluconate substituted) in C02-HC03--buffered Tyrode solution caused a large (0-5 pH units) reversible intracellular alkalosis. This alkalosis was inhibited by the anion exchange inhibitor DIDS but not by removal of external Na+ (NMG substituted).9. We conclude that the type I cells from neonatal rat carotid bodies possess at least three mechanisms for transmembrane acid equivalent transport: (i) Na+-H+ exchange which extrudes acid from the cell, (ii) another acid extrusion mechanism which requires both external Na+ and bicarbonate and which is DIDS inhibitable, and (iii) a Na+-independent Cl--HCO3-exchange which probably mediates a background acid influx.

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Phenoxybenzamine more effectively prevents alpha-adrenoreceptor mediated spasm of the human radial artery than papaverine. It is also less harmful to the endothelium.

SUMMARY1. The role of carbonic anhydrase near the medullary chemoreceptors has been investigated in the cat. Vertebral artery injections have been used to cause abrupt changes in respiration as a result of changes in the activity of medullary chemoreceptors.2. Injections of 100 % C02-saline were used to stimulate respiration and of Tris or alkalinized albumin solution to cause a reduction in respiration.3. The injections gave rapid effects. We studied the effect on these of benzolamide (1-4 mg/kg I.v.) a carbonic anhydrase inhibitor which does not easily cross the blood-brain barrier and acetazolamide (50 mg/kg i.v.) an inhibitor which crosses the barrier more easily.4. The effects of Tris were much reduced after benzolamide. Even addition of benzolamide to the injected Tris or albumin was sufficient to reduce their effects.5. The effects of C02-saline were reduced only after acetazolamide i.v. Whereas addition of carbonic anhydrase to injected Tris potentiated the effects on respiration, after acetazolamide this potentiation was much less marked.6. It is concluded that carbonic anhydrase acts in the region of the medullary chemoreceptors at two sites: (a) outside the blood-brain barrier, probably at the luminal surface of the capillary endothelium, where it may act on plasma buffers, and (b) inside the barrier, in association with the chemoreceptors, where it may accelerate C02/pH equilibration.