Cardiac microvascular endothelial cells express a functional Ca2+ -sensing receptor

The mechanism whereby extracellular Ca 2+ exerts the endothelium-
dependent control of vascular tone is still unclear.
In this study, we assessed whether cardiac microvascular endothelial
cells (CMEC) express a functional extracellular Ca 2+ -
sensing receptor (CaSR) using a variety of techniques. CaSR
mRNA was detected using RT-PCR, and CaSR protein was
identified by immunocytochemical analysis. In order to assess
the functionality of the receptor, CMEC were loaded
with the Ca 2+ -sensitive fluorochrome, Fura-2/AM. A number
of CaSR agonists, such as spermine, Gd 3+ , La 3+ and neomycin,
elicited a heterogeneous intracellular Ca 2+ signal, which
was abolished by disruption of inositol 1,4,5-trisphosphate
(InsP 3 ) signaling and by depletion of intracellular stores with
cyclopiazonic acid. The inhibition of the Na + /Ca 2+ exchanger
upon substitution of extracellular Na + unmasked the Ca 2+
signal triggered by an increase in extracellular Ca 2+ levels.
Finally, aromatic amino acids, which function as allosteric activators
of CaSR, potentiated the Ca 2+ response to the CaSR
agonist La 3+ . These data provide evidence that CMEC express
CaSR, which is able to respond to physiological agonists
by mobilizing Ca 2+ from intracellular InsP 3 -sensitive
stores.