Capillary pericytes mediate coronary no-reflow after myocardial ischaemia.

After cardiac ischaemia, a prolonged decrease of coronary microvascular perfusion often occurs even after flow is restored in an upstream artery. This 'no-reflow' phenomenon worsens patient prognosis. In the brain, after stroke, a similar post-ischaemic 'no-reflow' has been attributed to capillary constriction by contractile pericytes.

What is a pericyte?

Pericytes, spatially isolated contractile cells on capillaries, have been reported to control cerebral blood flow physiologically, and to limit blood flow after ischaemia by constricting capillaries and then dying. Paradoxically, a recent paper dismisses the idea of pericytes controlling cerebral blood flow, despite confirming earlier data showing a role for pericytes. We show that these discrepancies are apparent rather than real, and depend on the new paper defining pericytes differently from previous reports.

A role for pericytes in coronary no-reflow.

Despite efforts to restore tissue perfusion after myocardial infarction, coronary no-reflow--a failure to achieve adequate reperfusion of the cardiac microcirculation--is a common complication, which correlates with an increased incidence of death and disability. The treatment of ischaemic stroke is also plagued by no-reflow and, in the brain, a major cause of this phenomenon has been shown to be contractile microvascular pericytes irreversibly constricting capillaries and dying. We propose that cardiac pericytes, which are the second most-common cell type in the heart, impede reperfusion of coronary capillaries in a similar fashion to those in the brain after a stroke.

Capillary pericytes regulate cerebral blood flow in health and disease.

Increases in brain blood flow, evoked by neuronal activity, power neural computation and form the basis of BOLD (blood-oxygen-level-dependent) functional imaging. Whether blood flow is controlled solely by arteriole smooth muscle, or also by capillary pericytes, is controversial. We demonstrate that neuronal activity and the neurotransmitter glutamate evoke the release of messengers that dilate capillaries by actively relaxing pericytes.

Imaging pericytes and capillary diameter in brain slices and isolated retinae.

The cerebral circulation is highly specialized, both structurally and functionally, and it provides a fine-tuned supply of oxygen and nutrients to active regions of the brain. Our understanding of blood flow regulation by cerebral arterioles has evolved rapidly. Recent work has opened new avenues in microvascular research; for example, it has been demonstrated that contractile pericytes found on capillary walls induce capillary diameter changes in response to neurotransmitters, suggesting that pericytes could have a role in neurovascular coupling.