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Short‐term desensitization of the histamine H1 receptor in human HeLa cells: involvement of protein kinase C dependent and independent pathways

1 In this study we have investigated the effects of short‐term exposure of cells to histamine on the subsequent H1 receptor responsiveness in HeLa cells, using Ca2+ fluorescence microscopy and video digital imaging. 2 In HeLa cells, histamine (100 μm) induces an immediate H1 receptor‐mediated biphasic elevation of the intracellular Ca2+ concentration ([Ca2+]i) (basal [Ca2+]i: 81 ± 30 nm, histamine‐induced Ca2+ response: first phase: 1135 ± 79 nm; second phase: 601 ± 52 nm, n = 11). 3 The histamine H1 receptors on HeLa cells are readily susceptible to desensitization since repetitive exposure of the same group of cells to histamine (100 μm) markedly affected the release and influx component of the induced Ca2+ response (second application of histamine: first phase: 590 ± 92 nm, second phase: 279 ± 47 nm; third application of histamine: first phase: 454 ± 127 nm, second phase: 240 ± 45 nm, n = 6). Video digital imaging revealed an increase in the lag time between stimulation and monitoring of the Ca2+ response and a reduced increase in [Ca2+]i after desensitization with histamine. 4 Neither the release component of the ATP response (50 μm) nor the caffeine (3 mm)‐induced Ca2+ release were found to be affected by desensitization with 100 μm histamine. However, the second phase of the ATP response was significantly reduced after desensitization with histamine (control cells: 516 ± 33 nm; desensitized cells: 331 ± 96 nm, n = 4, P < 0.05). 5 Activation of protein kinase C (PKC) by phorbol‐12‐myristate‐13‐acetate was found to inhibit the histamine as well as ATP‐induced Ca2+ response in a dose‐dependent manner. 6 In PKC downregulated cells the second phase of the histamine‐induced Ca2+ response was significantly elevated, indicating the involvement of PKC in the negative feedback on the Ca2+ influx (control cells: second phase: 601 ± 52 nm (n = 11); PKC downregulated cells: second phase: 890 ± 90 nm, n = 10, P < 0.05). 7 Homologous desensitization of H1 receptor responsiveness was still observed in PKC downregulated cells, implying the rapid activation of a regulatory mechanism other than PKC. 8 Based on our experimental data we suggest that short‐term desensitization of the histamine H1 receptor evolves from two different processes: a selective reduction of the histamine‐induced Ca2+ release, mediated by a PKC‐independent pathway, and a non‐selective inhibition of the receptor‐mediated Ca2+ influx activated by a PKC‐dependent pathway.