Histamine-stimulated increases in intracellular calcium in the smooth muscle cell line, DDT1MF-2

Suspensions of undifferentiated cultured vas deferens smooth muscle cells (DDT1MF-2) were loaded with the calcium-sensitive fluorescent dye fura-2. Exposure to histamine elicited a rapid and maintained increase in intracellular free calcium ([Ca2+] i) with an EC50 of 1.3 +/- 0.7 x 10(-5) M. The initial rise is a consequence of calcium release from intracellular stores, whereas the maintained or plateau phase, which is dependent upon the presence of extracellular calcium, is associated with calcium influx. Experiments in nominally Ca(2+)-free buffer attenuated the initial rise in [Ca2+]i (i.e. peak height) and virtually abolished the plateau phase. Re-addition of 2 mM Ca2+ (during experiments performed in nominally Ca(2+)-free buffer) resulted in a return of the plateau phase. Pretreatment with the H1-antagonist mepyramine (100 nM; Kd = 1.0 +/- 0.4 nM, N = 3) completely blocks the response to histamine, whereas tiotidine (2 microM; H2-antagonist) had no effect. In conclusion, the present data would suggest that functional H1-receptors found in hamster vas deferens smooth muscle cells are typical of the "classical" H1-receptor in both its control of intracellular Ca2+ and sensitivity to antagonism by mepyramine.