Rm. Helliwell et Wa. Large, DUAL EFFECT OF EXTERNAL CA2-ACTIVATED CATION CURRENT IN RABBIT PORTAL-VEIN SMOOTH-MUSCLE CELLS( ON NORADRENALINE), Journal of physiology, 492(1), 1996, pp. 75-88
1. The influence of divalent cations was investigated on the noradrena
line-activated nonselective cation current (I-cat) in freshly disperse
d smooth muscle cells of rabbit portal vein. 2. Reduction of external
calcium concentration ([Ca2+](o)) from 1.5 mM to 'test' levels of 100-
500 mu M during stimulation of I-cat produced a sustained potentiation
of I-cat, which indicates an inhibitory action of Ca-o(2+). With 'tes
t' concentrations lower than 100 mu M Ca-o(2+) the amplitude of I-cat
was initially potentiated and then the current subsequently declined.
Under these conditions re-addition of 1.5 mM Ca-o(2+) transiently acti
vated I-cat which demonstrates an additional facilitatory action of [C
a2+](o). The half-maximal [Ca2+](o) for the facilitatory and inhibitor
y action on I-cat was 6 and 400 mu M, respectively. 3. When [Ca2+](o)
was decreased to very low levels (< 10 nM) by inclusion of 1 mM BAPTA
in nominally Ca2+-free external solution, I-cat could still be evoked
by noradrenaline. This suggested that Ca2+ is not obligatory for activ
ation of I-cat but merely modulatory. 4. In 1.5 mM Ca-o(2+) the curren
t-voltage (I-V) relationship of I-cat was S-shaped with a reversal pot
ential (E(r)) of about +9 mV. The I-V characteristics and E(r) of the
potentiated current in 200 mu M Ca-o(2+) were not markedly different f
rom those in 1.5 mM Ca-o(2+). 5. During the noradrenaline-induced I-ca
t removal of external Mg2+ caused a small increase in the amplitude of
I-cat which was not as great as that seen on reduction of [Ca2+](o) a
nd there was no facilitatory effect of Mg-o(2+). 6. These results indi
cate that external Ca2+ has a dual action on I-cat, both facilitating
and inhibiting the current. This dual action is not shared with extern
al Mg2+.