Mudpuppy parasympathetic neurons exhibit spontaneous miniature 
hyperpolarizations (SMHs) that are generated by potassium currents, which are 
spontaneous miniature outward currents (SMOCs), flowing through clusters of 
large conductance voltage- and calcium (Ca(2+))-activated potassium (BK) 
channels. The underlying SMOCs are initiated by a Ca(2+)-induced Ca(2+) release 
(CICR) mechanism. Perforated-patch whole cell voltage recordings were used to 
determine whether activation of SMHs contributed to action potential (AP) 
repolarization or affected the latency to AP generation. Blockade of BK channels 
by iberiotoxin (IBX, 100 nM) slowed AP repolarization and increased AP duration. 
Treatment with omega-conotoxin GVIA (3 microM) or nifedipine (10 microM) to 
inhibit Ca(2+) influx through N- or L-type voltage-dependent calcium channels 
(VDCCs), respectively, also decreased the rate of AP repolarization and 
increased AP duration. Elimination of CICR by treatment with either thapsigargin 
(1 microM) or ryanodine (10 microM) produced no significant change in AP 
repolarization or duration. Blockade of BK channels with IBX and inhibition of 
N-type VDCCs with omega-conotoxin GVIA, but not inhibition of L-type VDCCs with 
nifedipine, decreased the latency of AP generation. A decrease in latency to AP 
generation occurred with elimination of SMHs by inhibition of CICR following 
treatment with thapsigargin. Ryanodine treatment decreased AP latency in three 
of six cells. Apamin (100 nM) had no affect on AP repolarization, duration, or 
latency to AP generation, but did decrease the hyperpolarizing afterpotential 
(HAP). Inhibition of L-type VDCCs by nifedipine also decreased HAP amplitude. 
Inhibition of CICR by either thapsigargin or ryanodine treatment increased the 
number of APs generated with long depolarizing current pulses, whereas exposure 
to IBX or omega-conotoxin GVIA depressed excitability. We conclude that CICR, 
the process responsible for SMH generation, represents a unique mechanism to 
modulate the response to subthreshold depolarizing currents that drive the 
membrane potential toward the threshold for AP initiation but does not 
contribute to AP repolarization. Subthreshold depolarizations would not activate 
sufficient numbers of VDCCs to allow Ca(2+) influx to elevate [Ca(2+)](i) to the 
extent needed to directly activate nearby BK channels. However, the elevation in 
[Ca(2+)](i) is sufficient to trigger CICR from ryanodine-sensitive Ca(2+) 
stores. Thus CICR acts as an amplification mechanism to trigger a local 
elevation of [Ca(2+)](i) near a cluster of BK channels to activate these 
channels at negative levels of membrane potential.