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Subplasmalemmal ryanodine-sensitive Ca²⁺ release contributes to Ca²⁺-dependent K⁺ channel activation in a human umbilical vein endothelial cell line

Maud Frieden and Wolfgang F. Graier

1. The whole-cell configuration of the patch clamp technique was used to assess the involvement of ryanodine-sensitive Ca²⁺ release (RsCR) in histamine-activated Ca²⁺-dependent K⁺ (K_Ca) channels in the human umbilical vein endothelial cell line EA.hy926.

2. Histamine (10 µM) induced a transient outward current that reached 18·9 ± 5·5 pA pF^-1 at +20 mV. This current was diminished by 1 mM tetraethylammonium or 50 nM iberiotoxin, by 90 % and 80 %, respectively, suggesting that this current results from the stimulation of large-conductance K_Ca (BK_Ca) channels.

3. In about 50 % of the cells tested, stimulation of RsCR with 200 nM ryanodine initiated a small outward current that was also sensitive to iberiotoxin.

4. Following the ryanodine-mediated RsCR, the potency of 10 µM histamine to activate K_Ca channels was reduced by about 60 %. In agreement, an inhibition of RsCR with 25 µM ryanodine diminished K_Cacurrent in response to histamine by about 70 %.

5. The effect of 100 µM histamine on K_Ca channel activity was not reduced by previous RsCR with 200 nM ryanodine, or by an inhibition of RsCR by 25 µM ryanodine.

6. Histamine (10 µM)-induced Ca²⁺ elevation was reduced by 30 % following ryanodine-mediated RsCR, whereas no inhibition occurred in the case of 100 µM histamine stimulation.

7. In cells treated with 10 µM nocodazole for 16 h to collapse the superficial endoplasmic reticulum, 200 nM ryanodine failed to initiate any K_Ca current. Furthermore, the inhibitory effect of previous RsCR on 10 µM histamine-induced K_Ca current was not obtained in nocodazole-treated cells.

8. Our data suggest that during moderate cell stimulation (10 µM histamine), subplasmalemmal RsCR greatly contributes to the activation of K_Ca channels in endothelial cells. Thus, the function of the subplasmalemmal Ca²⁺ control unit (SCCU) described previously must be extended as a regulator for K_Ca channels.

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