Atrial Function/drug effects; Atrial Natriuretic Factor; Heart Atria; Myocardial Contraction/drug effects; Receptors, Somatostatin; Somatostatin/pharmacology; Animals; Atrial Natriuretic Factor/antagonists & inhibitors; Atrial Natriuretic Factor/metabolism; Blotting, Western; Cyclic AMP-Dependent Protein Kinases/antagonists & inhibitors; Cyclic AMP-Dependent Protein Kinases/metabolism; Diabetes Mellitus, Experimental/physiopathology; Extracellular Fluid/drug effects; Heart Atria/drug effects; Heart Atria/metabolism; Hemodynamics/drug effects; Imines/pharmacology; Male; Myocardial Contraction/physiology; Oligopeptides/pharmacology; Organ Culture Techniques; Peptides, Cyclic/pharmacology; Polymerase Chain Reaction; Protein Kinase Inhibitors/pharmacology; RNA, Messenger/analysis; Rats; Rats, Sprague-Dawley; Receptors, Somatostatin/agonists; Receptors, Somatostatin/antagonists & inhibitors; Receptors, Somatostatin/metabolism; Signal Transduction/drug effects; Somatostatin/antagonists & inhibitors