Overexpression of alpha1B-adrenergic receptor induces left ventricular dysfunction in the absence of hypertrophy

The stimulation of cardiac alpha1-adrenergic receptors (AR) modulates the heart's inotropic response and plays a role in the induction of cardiomyocyte hypertrophy. We have analyzed transgenic mouse lines overexpressing a wild-type alpha1B-AR specifically in the heart. Basal level systolic and diastolic left ventricular (LV) contractile function was depressed both in the anesthetized closed-chest mouse and the perfused working-heart preparation. Intrinsic LV function was further characterized under controlled preload and afterload conditions using the perfusion model. Contractile parameters were restored by chronic treatment with the alpha-AR antagonist prazosin. In ventricular function curves, the load-dependent force increases (length-tension effects) remained intact, although the transgenic curve was shifted to lower levels. The basal level contractile deficits were paralleled by a decrease in calcium transients in isolated LV cardiomyocytes. LV function comparable to controls was restored by isoproterenol stimulation. The physiological changes occurred in the absence of cardiomyocyte hypertrophy. This transgenic model will be useful for studying the potential role of alpha1-AR in cardiac contractility and hypertrophy.

Receptors involved in nerve-mediated vasoconstriction in small arteries of the rat hepatic mesentery

1. We have investigated the neurotransmitters and receptor subtypes involved in nerve-mediated vasoconstriction in small arteries of the rat hepatic mesentery. 2. A dense sympathetic innervation was demonstrated using catecholamine histochemistry and antibodies against the synaptic vesicle protein synaptophysin. 3. Reverse transcription-polymerase chain reaction (RT-PCR) demonstrated very strong expression of the alpha1A-adrenergic, neuropeptide Y (NPY) Y1, P2X1- and P2X4-purinergic receptors, moderate expression of the alpha2B-adrenergic receptor and the purinergic P2X5- and P2X7-receptors and weak expression of the alpha1B-, alpha1D-, alpha2A- and alpha2C-adrenergic receptors and the P2X2- and P2X3-purinergic receptors. NPY2 and P2X6 receptor expression was absent. 4. Electrical field stimulation (10 Hz, 10 s) produced contractions which were abolished by tetrodotoxin (10(-6) M) and/or guanethidine (GE, 5 x 10(-6) M) and a combination of benextramine (10(-5) M) and alpha,beta-methylene ATP, (alpha,beta-mATP, 3 x 10(-6) M) or PPADS (10(-5) M). Selective alpah1-adrenergic receptor antagonists showed the potency order of prazosin > WB-4101 > 5-methyl-urapidil > BMY 7378. Yohimbine (10(-8) M, 10(-7) M), alpha,beta-mATP (3 x 10(-6) M) and PPADS (10(-5) M) each enhanced the response to nerve stimulation. 5. Some experiments demonstrated a slow neurogenic contraction which was abolished by GE or the selective NPY1 receptor antagonist 1229U91 (6 x 10(-7) M). 6. We conclude that nerve-mediated vasoconstriction results from the activation of postsynaptic alpha,beta-adrenergic and P2X-purinergic receptors and under some conditions, NPY1 receptors. Neurotransmitter release is modulated by presynaptic alpha2-adrenergic receptors and possibly also P2X-purinoceptors. The major postsynaptic subtypes involved were well predicted by mRNA expression as measured by RT-PCR, suggesting that this technique may be a useful adjunct to studies aimed at identifying functional receptor subtypes.

Proximal nephron Na+/H+ exchange is regulated by alpha 1A- and alpha 1B-adrenergic receptor subtypes

Activation of alpha 1-adrenergic receptors (alpha 1-AR) increases Na+/H+ exchange (NHE) in proximal tubule. NHE mediates the majority of active Na+ absorption in the proximal tubule. Three alpha 1-AR subtypes have been detected in kidney by molecular and binding techniques. We detected message for all three alpha 1-AR subtypes in mouse proximal tubule cells through reverse transcription-polymerase chain reaction and Northern analysis. To determine the alpha 1-AR subtypes that regulate NHE in mouse proximal tubule cells, two strategies were used: (i) antisense oligodeoxynucleotides (ODNs) to selectively inhibit expression of alpha 1A-, alpha 1B-, and alpha 1D-AR subtypes and (ii) subtype-selective alpha 1-AR antagonists. Streptolysin-O permeabilization was used to introduce antisense and sense ODNs into cells three times over 72 hr. Western blot analysis of membranes prepared from cells treated with alpha 1B-AR antisense ODN demonstrated that alpha 1B-AR protein expression was reduced by 90% at 72 hr compared with control or sense ODN treatments. Functional regulation of NHE by alpha 1-ARs was determined by alpha 1-AR agonist changes in intracellular pH (pHi) in cells grown on coverslips and loaded with 2',7'-bis(2-carboxyethyl)-5(6)carboxyfluorescein-acetoxymethyl ester. Antisense ODNs for alpha 1B-AR significantly reduced phenylephrine (PHE)-induced maximal changes in pHi by 49%. The PHE-induced changes in pHi observed in cells treated with alpha 1A-AR antisense ODNs was reduced by 42%. The selective alpha 1A-AR antagonist WB-4101 and the alpha 1B-AR antagonist spiperone reduce PHE-induced pHi increases to a comparable extent. No significant changes in pHi were observed with cells treated with alpha 1D-AR antisense ODNs or the alpha 1D-AR antagonist BMY 7378 compared with untreated cells. Combined treatment with alpha 1A- and alpha 1B-AR antisense ODNs and antagonists additively inhibits PHE-induced delta pHi by 90%. We conclude that alpha 1A and alpha 1B-AR but not alpha 1D-ARs regulate NHE in proximal tubule cells.