Transport of beta-adrenergic antagonists in the absence of beta-adrenergic receptors in rat pituitary tumor cells

The quantal secretion of catecholamines is impaired by the accumulation of beta-adrenoceptor antagonists into chromaffin cell vesicles

BACKGROUND AND PURPOSE

The delayed onset of certain effects of antagonists of beta-adrenoceptors (beta-blockers), such as lowering arterial blood pressure (several days), cannot be explained solely by their effects on beta-adrenoceptors, an action that occurs within minutes. Although several mechanisms have been proposed, none of them explain this temporal delay. This work aimed at providing a new explanation based on the interference of these drugs with the functional accumulation of catecholamines within neurosecretory vesicles.

EXPERIMENTAL APPROACH

We used the simultaneous on-line monitoring of catecholamine and labetalol release from bovine isolated chromaffin cells and from rat perfused adrenal glands, as well as single cell amperometry, intracellular electrochemistry, patch amperometry and HPLC.

KEY RESULTS

Using amperometry, three beta-blockers, labetalol, atenolol and propranolol, reduced the quantal size of secretory events in chromaffin cells, accompanied by a slowing down of exocytosis. By patch amperometry, we found that treatment with beta-blockers also increases the chromaffin vesicle volume, thereby creating a functional dilution of catecholamines. Experiments with intracellular electrochemistry show that vesicles cannot uptake new catecholamines. There was progressive accumulation of labetalol in secretory vesicles of bovine adrenal chromaffin cells, and this beta-blocker was co-released with catecholamines from rat and bovine chromaffin tissues.

CONCLUSIONS AND IMPLICATIONS

We propose that beta-blockers are progressively concentrated into sympathetic secretory vesicles, and interfere with the storage of catecholamines and are co-released with the natural transmitters, resulting in a decrease in the sympathetic tone. This could explain the delayed onset of the hypotensive effects of beta-blockers.

Stereoselective increase of plasma concentrations of the enantiomers of propranolol and atenolol during exercise

OBJECTIVE

In vitro studies have shown that, like catecholamines, both propranolol and atenolol are taken up by and released from adrenergic cells. We performed this study to investigate whether this may also play a role in humans and whether stereoselective aspects are important.

METHODS

This was a randomized, double-blind, placebo-controlled, crossover study of two groups of 12 healthy volunteers. Subjects received single oral doses of 80 mg (R,S)-, 40 mg (R)-, and 40 mg (S)-propranolol; 100 mg (R,S)-, 50 mg (R)-, and 50 mg (S)-atenolol; and placebo at intervals of 1 week. Exercise was performed at 4 and 9 hours after drug intake, and blood samples were taken before and at the end of each exercise period. The plasma concentrations of the (R)- and (S)-enantiomers of propranolol and atenolol, as well as those of epinephrine and norepinephrine, were determined by HPLC.

RESULTS

Effects of exercise on the plasma levels of the enantiomers of propranolol and atenolol were similar. When the optically pure enantiomers were administered, exercise caused a marked and significant increase of the plasma concentrations of the (S)- but not of the (R)-enantiomers. When the drugs were administered in the racemic form, the plasma levels of both the (R)- and (S)-enantiomers were elevated to the same extent. The increase of norepinephrine levels during exercise was more pronounced than that of epinephrine and paralleled that of the (S)-enantiomers of the beta-blockers.

CONCLUSION

Bearing the in vitro data in mind, we conclude that (S)-propranolol and (S)-atenolol are taken up into and released from adrenergic cells together with norepinephrine during exercise. The reason why the plasma concentrations of (R)-propranolol and (R)-atenolol are increased only during exercise in the presence of the corresponding (S)-enantiomers remains to be determined.

125I-iodopindolol binding to frog esophageal peptic cells. Detection of amine uptake and beta-adrenergic receptors coupled to pepsinogen secretion

The beta-adrenergic receptors (beta-ARs) coupled to pepsinogen secretion on frog esophageal peptic cells have been compared to frog erythrocyte beta-ARs using the radioligand 125I-iodopindolol (125I-PIN). 125I-PIN binding to intact peptic cells was time and temperature dependent. Saturation and competition experiments established that a large component of this binding represented radioligand uptake, which was energy dependent, pH sensitive, Na+ independent, and inhibited by agents that depress cellular ATP or disrupt proton gradients. This uptake system, which was absent from frog erythrocytes, appeared similar to that recently described for a number of mammalian cells. 125I-PIN bound to a single class of sites on peptic cell homogenates with a KD = 64 (+/- 5) pM. Binding to cell homogenates and a proportion of the binding to intact cells was inhibited by beta-agonists and antagonists with pharmacological characteristics similar to typical beta 2-ARs of frog erythrocytes. The number of beta-ARs in these peptic cell preparations was 1300 (+/- 240) sites/cell. Isolated peptic cells were poorly responsive to isoproterenol stimulation even in the presence of the phosphodiesterase inhibitor IBMX (3-isobutyl-1-methylxanthine). Pretreatment of cells with the phorbol ester TPA (12-O-tetra-decanoylphorbol-13-acetate) (100 nM) promoted isoproterenol stimulation of pepsinogen secretion. Catecholamine agonists stimulated pepsinogen secretion with an order of potency: isoproterenol greater than epinephrine much greater than norepinephrine, which was identical to that determined for inhibition of 125I-PIN binding. These findings indicate that frog peptic cells contain beta 2-ARs functionally coupled to pepsinogen secretion.

Carbostyril-based beta-adrenergic agonists: evidence for long lasting or apparent irreversible receptor binding and activation of adenylate cyclase activity in vitro

The interaction of the carbostyril derivatives 5-[2-[[1-(4-aminophenyl)-2-methyl-prop-2-yl]amino]-1-hydroxyethyl]- 8-hydroxycarbostyril (carbo-amine) and 5-[2-[[3-[4-(bromoacetamido)phenyl]-2-methylprop-2-yl]amino]-1- hydroxyethyl]-8-hydroxycarbostyril (carbo-Br) with the rat reticulocyte beta-adrenoreceptor system has been partially characterized. In the absence of a guanine nucleotide, the concentration of carbo-amine, carbo-Br and (-)isoprenaline that inhibited (-)-[125I]iodocyanopindolol ([125I]CYP) binding by 50% (IC50) was 5.9 +/- 0.2, 3.3 +/- 0.3 and 49 +/- 3 nM, respectively. In the presence of a guanine nucleotide, the IC50 values were carbo-amine, 21 +/- 0.6 nM; carbo-Br, 7.6 +/- 0.3 nM and (-)isoprenaline, 813 +/- 66 nM. Preincubation of membranes with either of the carbostyril congeners followed by washing reduced specific [125I]CYP binding capacity without changing the KD value for the remaining receptors. The beta-antagonist nadolol largely prevented the receptor reduction induced by the carbostyril compounds. Incubation of membranes for 18 h at 25 degrees C resulted in an 11% recovery of the carbo-amine-induced receptor loss and no recovery of the receptors lost by preincubation with carbo-Br. However, the carbo-amine induced receptor loss could be largely reversed (80%) by membrane heating at 45 degrees C whereas little reversal (less than 10%) was observed with the carbo-Br pretreated membranes. The concentration of carbo-amine, carbo-Br and (-)isoprenaline that stimulated half-maximal cAMP formation in reticulocyte membranes was 17.8 +/- 3.1, 8.2 +/- 2.1 and 241 +/- 17 nM, respectively, and all 3 agonists produced the same maximal response. Initial cAMP formation stimulated by the carbostyril derivatives and (-)isoprenaline was blocked by concurrent addition of propranolol after 7 min of incubation with either of the two carbostyril derivatives did not affect further cAMP production whereas with (-)isoprenaline further cAMP production was blocked.(ABSTRACT TRUNCATED AT 250 WORDS)

Uptake of imipramine in neurons cultured from rat cerebellum

Imipramine was taken up by rat cerebellar neurons in primary culture. The process was dependent on time, temperature and pH and was reduced in the presence of the adenosine triphosphatase (ATPase) inhibitor dicyclohexylcarbodiimide or in the absence of glucose. Uptake approached saturation at 100 microM imipramine where approximately 42 nmol of the drug accumulated intracellularly per mg cell protein. Propranolol, but not serotonin, competed for imipramine uptake and uptake was inhibited by the 'lysosomotropic' amine chloroquine and by the Na+/H+ ionophore monensin, both of which dissipate proton gradients. Neurons were fractionated on Percoll gradients and the fractions exposed to [3H]imipramine. MgATP-dependent accumulation of [3H]imipramine was found mainly in fractions enriched for dense lysosomes. We conclude that imipramine was taken up by cerebellar neurons in primary culture and accumulated at high concentrations in intracellular compartments.