Failure of filipin to detect cholesterol-rich domains in smooth muscle plasma membrane.

Using the cholesterol probe filipin and freeze-fracture electron microscopy, Montesano reported an apparent difference in cholesterol content between contiguous zones of the smooth muscle plasma membrane. In this membrane, microinvaginations termed caveolae are organized in band-like domains, which were highly sensitive to filipin (and therefore cholesterol-rich) compared with the surrounding non-caveolar inter-band regions (assumed to be cholesterol-poor). Using another cholesterol probe, the saponin tomatin, applied alone and after filipin treatment, we now present evidence that both the band and inter-band domains of the smooth muscle plasma membrane are cholesterol-rich. This leads to the conclusion that the lack of inter-band response to filipin is a false-negative cytochemical result.