Pharmacological curve fitting to analyze cutaneous adrenergic responses

Abstract

Although dose-response curves are commonly used to describe in vivo cutaneous α-adrenergic responses, modeling parameters and analyses methods are not consistent across studies. The goal of the present investigation was to compare three analysis methods for in vivo cutaneous vasoconstriction studies using one reference data set. Eight women (22 ± 1 yr, 24 ± 1 kg/m(2)) were instrumented with three cutaneous microdialysis probes for progressive norepinephrine (NE) infusions (1 × 10(-8), 1 × 10(-6), 1 × 10(-5), 1 × 10(-4), and 1 × 10(-3) logM). NE was infused alone, co-infused with NG-monomethyl-l-arginine (l-NMMA, 10 mM) or Ketorolac tromethamine (KETO, 10 mM). For each probe, dose-response curves were generated using three commonly reported analyses methods: 1) nonlinear modeling without data manipulation, 2) nonlinear modeling with data normalization and constraints, and 3) percent change from baseline without modeling. Not all data conformed to sigmoidal dose-response curves using analysis 1, whereas all subjects' curves were modeled using analysis 2. When analyzing only curves that fit the sigmoidal model, NE + KETO induced a leftward shift in ED(50) compared with NE alone with analyses 1 and 2 (F test, P < 0.05) but only tended to shift the response leftward with analysis 3 (repeated-measures ANOVA, P = 0.08). Neither maximal vasoconstrictor capacity (E(max)) in analysis 1 nor %change CVC change from baseline in analysis 3 were altered by blocking agents. In conclusion, although the overall detection of curve shifts and interpretation was similar between the two modeling methods of curve fitting, analysis 2 produced more sigmoidal curves.

Fig. 1.

Mean norepinephrine (NE) and NE plus Ketorolac tromethamine (KETO) dose-response curves with no data manipulation (A; analysis method 1), after constraining the Hill slope (B; analysis method 1 with fixed slope), and after normalizing and constraining the top and bottom of the curve (C; analysis method 2). Ketorolac shifts the ED₅₀ to the left with all analyses (A–C), indicating enhanced cutaneous adrenergic responsiveness with prostaglandin inhibition. Goodness of fit for curve fitting was unaffected by constraining the Hill slope (slope = 1) for NE alone (r² = 0.60 and 0.63, unconstrained and constrained, respectively) or NE + KETO (r² = 0.74 and 0.76, unconstrained and constrained, respectively). Parameters of the model curves are listed in Tables 1–3. P < 0.05.

Fig. 2.

Mean NE and NE plus N^G-monomethyl-l-arginine (l-NMMA) dose-response curves with no data manipulation (A; analysis method 1), after constraining the Hill slope (B; analysis method 1 with fixed slope), and after normalizing and constraining the top and bottom of the curve (C; analysis method 2). No statistical effect of l-NMMA was detected using any analyses. Parameters of the model curves are listed in Tables 1–3.

Fig. 3.

Mean skin blood flow response expressed as a percent change from baseline cutaneous vascular conductance (%ΔCVC) during infusions of NE, NE + l-NMMA, and NE + KETO. All infusions resulted in a significant decrease in %ΔCVC (*P < 0.05 compared with baseline), with no differences in maximal vasoconstrictor capacity (at 1 × 10⁻³: NE −70.1 ± 8.1, NE + l-NMMA −62.9 ± 7.2, NE + KETO −68.7 ± 5.4%ΔCVC_BL). Co-infusions of l-NMMA (A) or KETO (B) did not alter %ΔCVC compared with NE alone, although the KETO intervention resulted in a trend toward a greater adrenergic response (P = 0.08).