Catfish cone horizontal cells contain a voltage-gated
L-type calcium channel that is modulated by activation
of metabotropic glutamate receptors (mGluRs). Activation
of group I mGluRs with the mGluR I agonist, (S)-3,5-dihydroxyphenylglycine
[(S) 3,5-DHPG], potentiated peak calcium current
amplitude, shifted the membrane potential corresponding
to peak current activity, and widened the calcium current's
activation range. In this study, we have examined the mechanisms
linking activation of the mGluRs with “up-regulation”
of calcium current activity. Under whole-cell voltage-clamp
conditions favoring expression of the L-type calcium current,
we provide evidence that activation of mGluRs initiate
the diacylglyceral (DG) second messenger pathway to activate
protein kinase C (PKC) and up-regulate calcium channel
activity. This evidence was based on results using a number
of PKC activators and inhibitors. PKC activators mimicked
the effect of (S) 3,5-DHPG on calcium current activity.
Up-regulation of the calcium channel by PKC activators
or (S) 3,5-DHPG was eliminated if PKC inhibitors were present.
These results also demonstrated that activation of group
I mGluRs were linked to a pertussis toxin sensitive G-protein.
When the GTP analog, guanosine 5-0-(3-thiotriphosphate
(GTPγS), was allowed to diffuse into voltage-clamp
cells, up-regulation of the calcium channel occurred and
mimicked the effect of (S) 3,5-DHPG. However, when pertussis
toxin (PTX) was allowed to diffuse into the cell along
with GTPγS, GTPγS failed to modulate calcium current
activity. IP3 (inositol 1,4,5 triphosphate)
is a second product produced by activation of group I mGluRs.
Once formed, IP3 can trigger calcium release
from IP3-sensitive intracellular stores. To
determine if the IP3 second messenger system
was involved in up-regulation of calcium channel, (S) 3,5-DHPG
was applied to voltage-clamped cone horizontal cells containing
different concentrations of the calcium buffer, EGTA. Low
concentrations of EGTA failed to buffer calcium released
from intracellular stores. In the presence of low EGTA
concentrations, (S) 3,5-DHPG's enhancement of the
calcium current amplitude was reduced. Inhibition of the
calcium current amplitude in low concentrations of EGTA
was eliminated in the presence of the intracellular calcium
store blocker, heparin. These results suggest that both
the DG and IP3 second messenger pathways are
involved in modulation of the voltage-gated calcium channel
in catfish cone horizontal cells. The DG pathway up-regulates
the voltage-gated calcium channel activity whereas calcium
released from IP3 intracellular stores inhibits
peak current amplitude.