AT1 receptors and L-type calcium channels: functional coupling in supersensitivity to angiotensin II in diabetic rats.
OBJECTIVE
The study was designed to investigate the role of calcium channels in enhanced angiotensin II (Ang II)-induced contraction in thoracic aortic rings from diabetic rats.


METHODS
Ang II-induced isometric tension was studied in thoracic aortic rings isolated from control or streptozotocin-induced (8 weeks) diabetic rats. Saturation binding studies at AT1 receptors and L-type calcium channels were performed using [3H] Ang II and [3H] PN200110, respectively. Ang II-induced calcium influx was studied in fura-2-loaded single vascular smooth muscle cells isolated from thoracic aorta of control and diabetic rats.


RESULTS
Ang II did not induce contraction in calcium-free Krebs. In the presence of extracellular calcium, increased Emax (mg/mm2) and pD2 to Ang II was observed in aortic rings from diabetic (795.54+/-38.19; 8.27+/-0.12) compared to control (230.09+/-25.45; 7.68+/-0.22) rats, respectively. Nimodipine but not verapamil or diltiazem dose-dependently blocked the Ang II-induced contractions in a noncompetitive manner and its -log IC50 was significantly lower in aortic rings from diabetic (8.81+/-0.10) compared to control (9.34+/-0.11) rats. The Ang II-induced increase in intracellular calcium levels was significantly enhanced (2.5-fold) in vascular smooth muscle cells from diabetic rats. AT1 receptor saturation binding with [3H] Ang II revealed a significantly higher affinity (nM) and Bmax (pmol/mg protein) in aortic vascular membrane preparation from diabetic (0.31+/-0.04; 64.18+/-2.4) compared to control (0.52+/-0.02; 47.81+/-2.8) rats, respectively, while L-type calcium channel saturation binding with [3H] PN200110 showed a higher affinity (nM) with no change in the Bmax (fmol/mg protein) in diabetic (0.74+/-0.08; 4.52+/-0.40) compared to control (1.49+/-0.32; 5.43+/-0.60) aortic membranes, respectively.


CONCLUSIONS
Our results suggest that Ang II-induced contraction is dependent on extracellular calcium, and enhanced functional coupling of AT1 receptors and DHP-sensitive L-type calcium channels results in supersensitivity to Ang II in thoracic aorta isolated from diabetic rats.
Arun
K H S
KH
Cardiovascular and Receptorology Laboratory, Department of Pharmacology and Toxicology, National Institute of Pharmaceutical Education and Research (NIPER), Sector-67, S.A.S. Nagar, Punjab-160062, India.
Kaul
C L
CL
Ramarao
P
P
D000804:Angiotensin II / Q000494:pharmacology*; D000818:Animals; D001013:Aorta, Thoracic; D002118:Calcium / Q000032:analysis / Q000378:metabolism / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020746:Calcium Channels, L-Type / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D003864:Depression, Chemical; D003921:Diabetes Mellitus, Experimental / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D066298:In Vitro Techniques; D017275:Isradipine / Q000494:pharmacology; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000378:metabolism*; D009553:Nimodipine / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D044140:Receptor, Angiotensin, Type 1 / Q000378:metabolism*
D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; D044140:Receptor, Angiotensin, Type 1; D000804:Angiotensin II; D009553:Nimodipine; D002118:Calcium; D017275:Isradipine