ALPHA-ADRENERGIC AGONIST AND ENDOTHELIN-1 INDUCED INTRACELLULAR CA2+ RESPONSE IN THE PRESENCE OF A CA2+ ENTRY BLOCKER IN CULTURED RAT VENTRICULAR MYOCYTES
Hw. Dejonge et al., ALPHA-ADRENERGIC AGONIST AND ENDOTHELIN-1 INDUCED INTRACELLULAR CA2+ RESPONSE IN THE PRESENCE OF A CA2+ ENTRY BLOCKER IN CULTURED RAT VENTRICULAR MYOCYTES, Cell calcium, 18(6), 1995, pp. 515-525
Previously we demonstrated that stimulation of cultured neonatal rat v
entricular myocytes by either alpha(1)-adrenergic agonist or endotheli
n-l resulted in a rapid formation of total inositolphosphates, althoug
h the levels of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetr
akisphosphate did not rise significantly. The aim of this study was to
examine whether stimulation by alpha(1)-adrenergic agonist and endoth
elin-1 could still elicit phosphatidylinositol cycle mediated intracel
lular Ca2+ mobilization in these cells. The intracellular free Ca2+ co
ncentration ([Ca2+](i)) was measured by single cell imaging dual wavel
ength fluorescence microscopy in Fura-2-loaded cardiomyocytes. The int
erference of agonist induced [Ca2+](i) responses by the beat to beat v
ariation of [Ca-i(2+) was prevented by arresting the cells with the Ca
2+ entry blocker diltiazem (10 mu M). The [Ca2+](i) response (expresse
d as % of baseline ratio of fluorescence intensities of Fura-P at 340
nm and 380 nm excitation wavelength), induced by phenylephrine (10(-4)
M) and endotherin-1 (10(-8) M) was small, up to 20% of baseline after
9-20 min. In contrast, Ca2+-influx induced by incubation in Na+-free
buffer caused a steep increase of [Ca2+](i) up to 150% of baseline aft
er 30 s. Analysis of single cells following stimulation with phenyleph
rine or endothelin-l showed heterogeneity with respect to a rise in Ca
2+](i). However, if rapid Ca2+-influx was induced by incubation in Na-free buffer, [Ca+](i) responses in individual myocytes occurred homog
eneously. It is concluded that the alpha(1)-adrenergic agonist and end
othelin-l induced [Ca2+](i) responses are delayed in time, small and q
uite heterogeneous among cells. The findings are in agreement with ear
lier observations which revealed no detectable overall increase of the
Ca2+ releasing inositolphosphates under these conditions and suggest
that other second messengers, such as 1,2-diacylglycerol, are involved
in the agonist mediated Ca2+ signals.