ENDOGENOUS PACEMAKER ACTIVITY OF RAT-TUMOR SOMATOTROPHS

1. Cells derived from a rat pituitary tumour (GC cell line) that conti nuously release growth hormone behave as endogenous pacemakers. In sim ultaneous patch clamp recordings and cytosolic Ca2+ concentration ([Ca 2+](1)) imaging, they displayed rhythmic action potentials (44.7 +/- 2 .7 mV, 178 +/- 40ms, 0.30 +/- 0.04 Hz) and concomitant [Ca2+], transie nts (374 +/- 57 nM, 1.0 +/- 0.2 s, 0.27 +/- 0.03 Hz). 2. Action potent ials and [Ca2+](1) transients were reversibly blocked by removal of ex ternal Ca2+, addition of nifedipine (1 mu M) or Ni2+ (40 mu M), but we re insensitive to TTX (1 mu M). An L-type Ca2+ current activated at -3 3.6 +/- 0.4 mV (holding potential (V-h), -40 mV), peaked at -1.8 +/- 1 .3 mV, was reduced by nifedipine and enhanced by S(+)-SDZ 202 791. A T /R-type Ca2+ current activated at -41.7 +/- 2.7 mV (V-h, -80 or -60 mV ), peaked at -9.2 +/- 3.0 mV, was reduced by low concentrations of Ni2 + (40 mu M) or Cd2+ (10 mu M) and was toxin resistant. Parallel experi ments revealed the expression of the class E calcium channel alpha 1-s ubunit mRNA. 3. The K+ channel blockers TEA (25 mM) and charybdotoxin (10-100 nM) enhanced spike amplitude and/or duration. Apamin (100 nM) also strongly reduced the after-spike hyperpolarization. The outward K + tail current evoked by a depolarizing step that mimicked an action p otential reversed at -69.8 +/- 0.3 mV, presented two components, laste d 2-3 s and was totally blocked by Cd2+ (400 mu M). 4. The slow pacema ker depolarization (3.5 +/- 0.4 s) that separated consecutive spikes c orresponded to a 2- to 3-fold increase in membrane resistance, was str ongly Naf sensitive but TTX insensitive. 5. Computer simulations showe d that pacemaker activity can be reproduced by a minimum of six curren ts: an L-type Ca2+ current underlies the rising phase of action potent ials that are repolarized by a delayed rectifier and Ca2+-activated K currents. In between spikes, the decay of Ca2+-activated K+ currents and a persistent inward cationic current depolarize the membrane, acti vate the T/R-type Ca2+ current and initiate a new cycle.