Abstract
We have studied histamine (HA)-evoked intracellular Ca 2+ release in single, freshly isolated myocytes from the guinea pig urinary bladder. Short applications of histamine (5 s) produced a thapsigargin (TG)-sensitive transient increase in intracellular calcium concentration ([Ca 2+] i). It was established that histamine and caffeine (Caff) released Ca 2+ from the same intracellular stores in these cells. Reducing the Ca 2+ content of internal stores by incubating cells with U-73343 or cyclopiazonic acid (CPA) inhibited the histamine-evoked Ca 2+ release in 69% and 60% of cells, respectively. Under these conditions, all cells released Ca 2+ in response to either caffeine or acetylcholine (ACh). However, decreasing internal Ca 2+ stores by removing external Ca 2+ inhibited histamine-induced Ca 2+ mobilization in only 22% of cells. A similar small fraction of cells was inhibited when sarcoplasmic reticulum (SR) Ca 2+ pumps were quickly blocked to avoid a significant reduction of luminal Ca 2+. In conclusion, lowering the luminal Ca 2+ content in combination with an impairment of the SR Ca 2+ pump activity significantly diminishes the ability of histamine to evoke an all-or-none intracellular Ca 2+ release.
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