HISTAMINE AND ATP MOBILIZE CALCIUM BY ACTIVATION OF H-1, AND P-2U RECEPTORS IN HUMAN LENS EPITHELIAL-CELLS

1. Cytoplasmic calcium concentration ([Ca2+](i)) of single superfused tissue-cultured human lens epithelial cells (HLEC) was monitored using the fluorescent dye fura-2; the resting values were low and stable for several hours ([Ca2+](i) = 96 +/- 20 nar; mean +/- S.D., n = 16). 2. Continuous superfusion with either ATP or histamine (0.1-10 mu M) produced regular oscillations in [Ca2+](i) that could be maintained for a short time in the absence of external calcium. 3. Short (30 s) pulses of histamine (0.1-100 mu M) induced a transient rise in [Ca2+](i), the time course of which was insensitive to the removal of external calcium. The rate of rise and the amplitude of the response were very sensitive to agonist concentration, whereas the rate of recovery was relatively constant. 4. The responses to long pulses of histamine (> 100 s) consisted of an initial transient followed by a maintained [Ca2+](i) which returned to baseline on removal of external calcium. 5. The kinetics of the responses to short and long pulses of ATP (0.1-100 mu M) were very similar to those of histamine and showed a similar sensitivity to the presence or absence of external calcium. 6. The histamine responses were abolished by triprolidine (1 mu M), but unaffected by ranitidine (1 mu M), indicating that an H-1 receptor subtype is activated by histamine. 7. The ATP responses were reversibly inhibited by suramin and the potency sequence for a range of agonists was ATP = UTP = ATP gamma S > ADP = GTP much greater than AMP = adenosine, indicating that activation of a P-2u receptor subtype was responsible for the increase in [Ca2+](i). 8. Both histamine and ATP responses were abolished by thapsigargin (100 nM), confirming that calcium release from intracellular stores was responsible for the initial peak of the response. Application of either agonist during the plateau phase of the thapsigargin response often led to a marked, but reversible, decline in [Ca2+](i), indicating the presence of a further, normally hidden, calcium regulatory factor associated with the presence of the agonist. 9. Maximal concentrations of either histamine or ATP totally emptied the calcium store as a subsequent application of the other agonist (or thapsigargin), in the absence of external calcium, failed to induce a further increase in the calcium signal.