The recent use of confocal microscopy showed for the first time that the nucleus plays an important role in excitation-contraction and secretion coupling of several excitable and nonexcitable cells. It was also suggested that, like the sarcolemma membrane, the nuclear membrane indeed possesses channels as well as receptors which could play a major role in modulating Ca 2+ influx and cytosolic Ca 2+ buffering capacity of the nucleus.
In this study, the mechanism(s) of Ca 2+ entry through the sarcolemma and the nuclear membrane during spontaneous contraction of heart cells and during sustained stimulation of ET-1 and Ang-II receptors were examined in cardiomyocytes as well as in human vascular smooth muscle and endothelial cells.
During spontaneous contraction of heart cells, the cytosolic Ca 2+ increase was immediately followed by a transient increase of nuclear Ca 2+ , both of which were blocked by the L-type Ca 2+ channel blocker apamine at a concentration of 100 nM.
Sustained stimulation of bradykinin, Ang-II, or ET-1 receptors induced a sustained increase of intracellular Ca 2+ which was mainly nuclear.
Using the cell attached patch clamp configuration, the potent voltage-dependent steady-state R-type Ca 2+ channel blocker PN200-110 (in the presence of the specific L-type Ca 2+ -blocker nifedipine) decreased the PAFinduced increase of the probability of opening the sarcolemmal steady-state resting voltage-dependent R-type Ca 2+ channel. The use of an Ang-II fluorescent probe and sarcolemmal perforated technique showed that Ang II receptors are also located at the nuclear membrane level. These results demonstrate that like the sarcolemmal membrane, the nuclear membrane possesses receptors as well as channels that could be a target for cytosolic free hormones, and thus also constituting a new target for drug action.