Characteristics of the Ca2+-dependent inhibition of cyclic AMP accumulation by histamine and thapsigargin in human U373 MG astrocytoma cells

1 Histamine, acting on H-1-receptors, caused a Ca2+-dependent inhibition of forskolin- and isoprenaline-induced cyclic AMP accumulation in monolayers of human U373 MG cells (IC50 1.3+/-0.3 mu M, maximum inhibition 66+/-3%). T he inhibition was not reversed by the protein kinase inhibitor K-252A. 2 Thapsigargin also inhibited cyclic AMP accumulation (IC50 6.0+/-0.3 nM, m aximum inhibition 72 +/- 2%). In the absence of extracellular Ca2+ 5 mu M t hapsigargin caused only a 12+2% inhibition of cyclic AMP accumulation. 3 The inhibitory effect of 100 nM thapsigargin on forskolin-stimulated cycl ic AMP accumulation was blocked by La3+ (best-fit maximum inhibition 81+/-4 %, IC50 125+/-8 nM). In contrast, the inhibitory action of 10 mu M histamin e was much less sensitive to reversal by 1 mu M La3+ (33+/-5% reversal, com pared with 78+/-6% reversal of the inhibition by thapsigargin measured conc urrently). However, in the presence of both thapsigargin and histamine the inhibition of cyclic AMP accumulation was reversed by 1 mu M La3+ to the sa me extent as the inhibition by thapsigargin alone. 4 Thapsigargin (5 mu M) + 1 mu M La3+ caused only a 20+/-1% inhibition of h istamine-stimulated phosphoinositide hydrolysis. 5 There was no indication from measurement of intracellular Ca2+ of any per sistent La3+ insensitive Ca2+ entry component activated by histamine. 6 The results provide evidence that Ca2+ entry is required for the inhibiti on by histamine and thapsigargin of drug-induced cyclic AMP accumulation in U373 MC astrocytoma cells. The differential sensitivity of the inhibitory action of the two agents to block by La3+ suggests that more than one pathw ay of Ca2+ entry is involved.