Calcium kinetics in vascular smooth muscle

Mechanisms of cerebral vasospasm in subarachnoid haemorrhage

Cerebrovascular spasm is a slowly developing constriction of the cerebral arteries, which frequently follows subarachnoid haemorrhage and is associated with considerable morbidity and mortality. The condition has been studied by use of models of subarachnoid haemorrhage in the whole animal and examination of isolated blood vessels or vascular smooth muscle cells in culture. The condition probably arises from the action of haemoglobin released from erythrocytes trapped in the subarachnoid clots, although the mechanism of action of haemoglobin remains uncertain. Systemic pharmacotherapy to avert or reverse vasospasm is still experimental.

Effects of hypoxia upon contractions evoked in isolated rabbit pulmonary artery by potassium and noradrenaline

1. Comparisons have been made between rabbit thoracic aorta and main pulmonary artery of the effects of hypoxia upon contractions evoked by noradrenaline (NA) and KCl (K+). 2. Contractions were evoked in cylindrical sections of pulmonary artery and aorta, mounted for isometric recording of tension, by NA and K+ (at ED80) in normoxia (PO2 110 mmHg) and hypoxia (PO2 23 or 7 mmHg). Contractions were also evoked in Ca2(+)-free conditions with EGTA to prevent influx of extracellular Ca2+. All contractions are expressed as a percentage of normoxic response in the presence of Ca2+. 3. Potassium-evoked contractions of aorta and pulmonary artery were reduced to a similar extent by both levels of hypoxia, to 85 and 92% respectively. As expected K(+)-evoked contractions were virtually abolished by Ca2(+)-free conditions. It is proposed that hypoxia has a small inhibitory effect upon contraction mediated by Ca2+ influx via voltage-operated Ca2+ channels. 4. In the aorta in the presence of Ca2+, hypoxia reduced NA-evoked contractions to 84% at PO2 23 mmHg and 34% at PO2 7 mmHg. In the absence of Ca2+, NA-evoked contractions reached 73% in normoxia, but only 43 and 21% at PO2 23 and 7 mmHg respectively. These results suggest that hypoxia reduces the component of contraction that is mediated by release of intracellular Ca2+ and possibly that mediated by agonist-induced Ca2+ influx. 5. In the pulmonary artery also, NA-evoked responses in the absence of Ca2+ were reduced from 60% in normoxia, to 49 and 38% at PO2 23 and 7 mmHg. But, in the presence of Ca2+, hypoxia potentiated NA-evoked contractions to 113 and 111% at PO2 23 and 7 mmHg respectively. It is proposed that in the pulmonary artery, hypoxia reduces the component of contraction mediated by release of intracellular Ca2+, but facilitates that mediated by extracellular Ca2+. Possible mechanisms are discussed.

An investigation of the actions of diltiazem on rat aorta exposed to acute hypoxia followed by re-oxygenation

1 The effects of diltiazem and removal of extracellular Ca2+ were examined on contractions, of the rat isolated aorta, to noradrenaline (NA) and high K+, during exposure to oxygenated conditions and hypoxia followed by re-oxygenation. 2 Exposure to hypoxia caused a similar reduction of contractile responses to NA and KCl, while re-oxygenation restored contractile activity. 3 Ca2+-free conditions abolished responses to KCl but a transient response to NA remained which was resistant to hypoxia. 4 Diltiazem produced similar reductions of responses to NA during both oxygenated conditions and hypoxia, whereas during re-oxygenation the effects of diltiazem upon responses to NA were enhanced. 5 Diltiazem produced a more pronounced reduction of responses to KCl than of responses to NA. However, the reduction of responses to KCl by diltiazem was not modified by the changes in PO2 examined in the present study. 6 The present study indicates that contractions of the rat aorta mediated by intracellular Ca2+ are resistant to the hypoxic conditions studied in the present investigation, whereas those responses mediated by an influx of Ca2+ are reduced. The increase in the contractile response to NA following re-oxygenation may result from an increased influx of extracellular Ca2+ since such responses show an enhanced sensitivity to diltiazem.