Phenylalanine-stimulated secretion of cholecystokinin is calcium dependent.

The secretion of cholecystokinin was examined in STC-1 cells, an intestinal cholecystokinin (CCK)-secreting cell line. Exposure to the amino acid L-phenylalanine increased release of CCK by 135%, 180%, and 251% of control levels after 15-min treatments with 5, 20, and 50 mM phenylalanine, respectively. L-Phenylalanine-induced secretion of CCK was inhibited by the calcium channel blocker diltiazem (10 microM). L-Phenylalanine (20 mM) also significantly increased cytosolic calcium levels in fura 2-acetoxymethyl ester (fura 2-AM)-loaded cells, and this increase was diltiazem sensitive. D-Phenylalanine, over the dose range of 5-50 mM, produced nonsignificant increases in CCK release. Treatment of STC-1 cells with 300 ng/ml of pertussis toxin for either 4 or 24 h did not significantly affect either basal release of CCK or L-phenylalanine-stimulated secretion. Patch-clamp recordings from cell-attached membrane patches showed a stimulation in calcium channel activity after L-phenylalanine. These results indicate that, in STC-1 cells, L-phenylalanine stimulates release of cholecystokinin via a calcium-dependent process.

Regulation of cholecystokinin secretion by bombesin in STC-1 cells.

Bombesin stimulates cholecystokinin (CCK) secretion, presumably by a direct effect on the intestinal CCK cell. The present objectives were to characterize bombesin-stimulated CCK release and to investigate the role of calcium in CCK secretion in an intestinal CCK-producing cell line (STC-1). Bombesin caused a dose-dependent release of CCK, which was reduced either in the absence of extracellular calcium or by calcium channel blockade, suggesting that influx of calcium is necessary for CCK secretion. Bombesin caused an increase in intracellular calcium concentration ([Ca2+]i) and increased efflux of 45Ca2+ from 45Ca(2+)-loaded cells. Radioligand binding studies and Northern analysis were consistent with the expression of a bombesin receptor. Thus bombesin stimulation of CCK release occurs via binding to a receptor and is dependent on increased [Ca2+]i. We propose that the STC-1 cell line may provide a useful model for studying the regulation of intestinal CCK secretion.

Regulation of cholecystokinin secretion by ATP-sensitive potassium channels.

The relationship of potassium channel activity to the secretion of cholecystokinin (CCK) was evaluated in STC-1 cells, an intestinal CCK-secreting cell line. Patch-clamp and 86Rb efflux studies showed that an ATP-sensitive potassium channel was endogenously expressed in STC-1 cells. Furthermore, channels are present in sufficient number to significantly modulate whole cell potassium permeability after either channel activation or closure with diazoxide (100 microM) or disopyramide (200 microM), respectively. Inhibition of channel activity with glucose (5-20 mM) was found to depolarize the plasma membrane, increase cytosolic calcium levels, and stimulate CCK release. Glucose-mediated release of CCK, as well as the increase in cytosolic calcium, was inhibited by the calcium channel blocker diltiazem (10 microM). It is concluded that intestinal secretion of CCK may be tonically controlled by activity of basally active ATP-sensitive potassium channels, and after inhibition of channel activity, calcium-dependent CCK secretion is stimulated.

Potassium channels regulate cholecystokinin secretion in STC-1 cells.

Following blockade of plasma membrane potassium channels with barium or tetraethylammonium chloride, release of cholecystokinin was increased in an intestinal cell line (STC-1). Treatment with calcium channel blockers inhibited barium- or TEA-induced secretion. Barium chloride also stimulated 45Ca efflux from STC-1 cells. Whole cell patch clamp recordings revealed a voltage-activated, L-type calcium current. We conclude that, inhibition of basally active potassium channels may depolarize STC-1 cells, producing activation of voltage-gated calcium influx pathways. Influx of calcium may lead to a release of intracellular calcium which stimulates cholecystokinin secretion.

Calcium-dependent regulation of cholecystokinin secretion and potassium currents in STC-1 cells.

Secretory and electrophysiological properties of STC-1 cells, a cholecystokinin-secreting cell line, were examined with a radioimmunoassay and patch-clamp recording techniques. Stimulation of cholecystokinin secretion was seen after exposure to agents anticipated to increase the level of intracellular calcium, including thapsigargin (8 microM), bombesin (50 nM), potassium-induced depolarization (50 mM), or after blockade of potassium channels with barium chloride (2 mM). The secretory effects of these agents were blocked by pretreatment with the calcium channel blocker diltiazem (1 microM). Whole cell patch-clamp recordings showed a hyperpolarizing shift in reversal potential after exposure to either thapsigargin (8 microM) or bombesin (50 nM) from a control value of -27 +/- 3 to -57 +/- 7 or -48 +/- 6 mV, respectively. This shift was in the direction of the reversal potential for potassium and was blocked by barium chloride (5 mM). Single-channel recordings from cell-attached membrane patches showed an inwardly rectifying potassium channel with channel open probability modulated by bombesin. These results indicate that in STC-1 cells a potassium current is increased by agents that stimulate CCK secretion, presumably by increasing the level of cytosolic calcium. STC-1 cells may serve as a model system to study the electrophysiological and secretory mechanisms involved in the release of cholecystokinin.

Linearity of the Frank-Starling relationship in the intact heart: the concept of preload recruitable stroke work.

The Frank-Starling relationship generally has been examined with filling pressure as the index of preload, resulting in a curvilinear function that plateaus at higher filling pressures. To investigate this relationship further in the intact heart, 32 dogs were chronically instrumented with left ventricular and pleural micromanometers and with regional (10 dogs) or global (22 dogs) ultrasonic dimension transducers. Seven days after implantation, left ventricular pressure and regional or global dimensions were recorded in the conscious state. After autonomic blockade, preload was varied by vena caval occlusion. Myocardial function was assessed by calculating regional or global stroke work, and preload was measured as end-diastolic segment length or chamber volume. The relationship between stroke work and either end-diastolic segment length or chamber volume (termed the preload recruitable stroke work relationship) was highly linear in every study (mean r = .97) and could be quantified by a slope (MW) and x-intercept (LW). Previous nonlinear relationships between stroke work and filling pressure seemed to reflect the exponential diastolic pressure-volume curve. Over the physiologic range of systolic arterial pressures produced by infusion of nitroprusside or phenylephrine, no significant change was observed in MW or LW in the normal dog. Calcium infusion increased both regional and global MW by 71 +/- 19% and 65 +/- 9%, respectively (p less than .02), with no significant change in LW. To normalize for ventricular geometry and heart rate, stroke work was computed from circumferential stress-strain data and converted to myocardial power output, which was then plotted against end-diastolic circumferential strain. This relationship also was highly linear, and the slope, Mmp (mW/cm3 of myocardium), is proposed as a potential measure of intrinsic myocardial performance independent of loading, geometry, and heart rate.