In diabetes significant changes in vascular function occur which are associated with increased frequency and severity of vascular disease. Diabetes causes elevation of protein kinase C (PKC) activity (Evcimen & King 2007), one of the main elements in sensitizing myofilaments to Ca²⁺ (Woodsome et al., 2001; Soloviev et al., 2005). We hypothesized that PKC-mediated increase in the Ca²⁺-sensitivity of myofilaments and/or agonist sensitivity in α-adrenoceptor/G-protein coupled pathways in vascular myocytes may be involved in diabetes-associated alteration of vascular contractility. Smooth muscle isometric tension measurements were preformed using either intact or chemically permeabilised (60 μM escin) isolated vascular rings obtained from thoracic aorta and tail artery of humanely-dispatched male Sprague-Dawley rats. 12-week streptozotocin-treated rats (diabetic – measured plasma glucose, MPG, 26.7±1.2 mM, n=16), and age-matched controls (MPG – 5.0±0.2 mM, n=7) were used. Aortic rings showed no difference in sensitivity to α1-adrenoceptor agonist phenylephrine (0.1 nM-1 mM) between control and diabetic rats (pD₂ (-log EC₅₀)=7.35±0.15, n=8 and 7.46±0.17, n=10, respectively; mean±S.D., unpaired Student’s t-test). Pretreatment of tissue with the potent PKC inhibitor, chelerythrine (1 μM), significantly shifted concentration-response curves to the right for aortic rings from both groups (pD₂=5.76±0.23, n=6, P<0.001 in control and 6.89±0.21, n=8, P<0.05 in diabetes, respectively). In contrast to aorta, diabetic tail artery showed significantly higher sensitivity to phenylephrine (pD₂=6.36±0.17, n=11) compared to control (pD₂=5.78±0.12, n=8, P<0.02). However, inhibition of PKC had no effect on agonist sensitivity in tail artery (control pD₂=5.47±0.23, n=11; diabetic pD₂=6.01±0.06, n=8). Furthermore, in permeabilized aorta rings no differences were found in smooth muscle myofilament Ca²⁺-sensitivity (pCa₅₀) between control and diabetic vessels (6.12±0.01 vs. 6.14±0.01, respectively, n=8), or following PKC inhibition (control 6.13±0.01, n=6; diabetic 6.14±0.01, n=9). Similar results were obtained in permiabilized rat tail artery rings where pCa₅₀ values were 5.96±0.02 in control (n=6) compared to 5.90±0.02 (n=6) in diabetic vessels. Chelerythrine had no effect on Ca²⁺-tension relationships (control 5.93±0.02, n=11; diabetic 5.81±0.12, n=9). Thus, in tail artery, but not in aorta, diabetes mellitus is associated with an increased sensitivity to phenylephrine. Moreover, in rat aorta there is significantly smaller chelerythrine-induced desensitization of the α1-adrenoceptor mediated contractions in diabetic vessels compared to control. These differences were unlikely to be due to differential myofilament Ca²⁺-sensitivity or its modulation by PKC.