More in vivo experimentation is needed in cardiovascular physiology

Phenylephrine-Induced Cardiovascular Changes in the Anesthetized Mouse: An Integrated Assessment of in vivo Hemodynamics Under Conditions of Controlled Heart Rate

Objective

Investigating the cardiovascular system is challenging due to its complex regulation by humoral and neuronal factors. Despite this complexity, many existing research methods are limited to the assessment of a few parameters leading to an incomplete characterization of cardiovascular function. Thus, we aim to establish a murine in vivo model for integrated assessment of the cardiovascular system under conditions of controlled heart rate. Utilizing this model, we assessed blood pressure, cardiac output, stroke volume, total peripheral resistance, and electrocardiogram (ECG).

Hypothesis

We hypothesize that (i) our in vivo model can be utilized to investigate cardiac and vascular responses to pharmacological intervention with the α₁-agonist phenylephrine, and (ii) we can study cardiovascular function during artificial pacing of the heart, modulating cardiac function without a direct vascular effect.

Methods

We included 12 mice that were randomly assigned to either vehicle or phenylephrine intervention through intraperitoneal administration. Mice were anesthetized with isoflurane and intubated endotracheally for mechanical ventilation. We measured blood pressure via a solid-state catheter in the aortic arch, blood flow via a probe on the ascending aorta, and ECG from needle electrodes on the extremities. Right atrium was electrically paced at a frequency ranging from 10 to 11.3 Hz before and after either vehicle or phenylephrine administration.

Results

Phenylephrine significantly increased blood pressure, stroke volume, and total peripheral resistance compared to the vehicle group. Moreover, heart rate was significantly decreased following phenylephrine administration. Pacing significantly decreased stroke volume and cardiac output both prior to and after drug administration. However, phenylephrine-induced changes in blood pressure and total peripheral resistance were maintained with increasing pacing frequencies compared to the vehicle group. Total peripheral resistance was not significantly altered with increasing pacing frequencies suggesting that the effect of phenylephrine is primarily of vascular origin.

Conclusion

In conclusion, this in vivo murine model is capable of distinguishing between changes in peripheral vascular and cardiac functions. This study underlines the primary effect of phenylephrine on vascular function with secondary changes to cardiac function. Hence, this in vivo model is useful for the integrated assessment of the cardiovascular system.

Characteristics and Research Techniques Associated with the Journal Impact Factor and Other Key Metrics in Pharmacology Journals

In the present age, there is intense pressure on researchers to publish their research in ‘high-impact factor’ journals. It would be interesting to understand the trend of research publications in the field of pharmacology by exploring the characteristics of research articles, including research techniques, in relation to the journal’s key bibliometrics, particularly journal impact factor (JIF), the seemingly most mentioned metric. This study aimed to determine the characteristics and research techniques in relation to research articles in pharmacology journals with higher or lower JIF values. A cross-sectional study was conducted on primary research journals under the ‘Pharmacology and Pharmacy’ category. Analysis of 768 original research articles across 32 journals (with an average JIF of 2.565 ± 0.887) demonstrated that research studies involving molecular techniques, in vivo experiments on animals, and bioinformatics and computational modeling were significantly associated with a higher JIF value of the journal in which such contributions were published. Our analysis suggests that research studies involving such techniques/approaches are more likely to be published in higher-ranked pharmacology journals.

Imaging sympathetic neurogenic Ca2+ signaling in blood vessels

We review the information that has been provided by optical imaging experiments directed at understanding the role and effects of sympathetic nerve activity (SNA) in the functioning of blood vessels. Earlier studies utilized electric field stimulation of nerve terminals (EFS) in isolated arteries and vascular tissues (ex vivo) to elicit SNA, but more recently, imaging studies have been conducted in vivo, enabling the study of SNA in truly physiological conditions. Ex vivo: In vascular smooth muscle cells (VSMC) of isolated arteries, the three sympathetic neurotransmitters, norepinephrine (NE), ATP and neuropeptide Y (NPY), elicit or modulate distinct patterns of Ca²⁺ signaling, as revealed by confocal imaging of exogenous fluorescent Ca²⁺ indicators. Purinergic junctional Ca²⁺ transients (jCaTs) arise from Ca²⁺ influx during excitatory junction potentials (eJPs), and are associated with the initial neurogenic contraction. Adrenergic Ca²⁺ waves and oscillations cause contraction while SNA-induced endothelial Ca²⁺ 'pulsars' cause relaxation. In vivo: optical biosensor mice, which express genetically encoded Ca²⁺ indicators (GECI's) specifically in smooth muscle, combined with non-invasive imaging techniques has enabled imaging SNA-induced Ca²⁺ signaling and arterial diameter in vivo. SNA induces Ca²⁺ oscillations in intact arteries. [Ca²⁺] of arterial smooth muscle cells increased in hypertension, in association with increased SNA. High resolution imaging has revealed local sympathetic, neurogenic Ca²⁺ signaling within smooth muscle and endothelial cells of the vasculature. The ongoing development of in vivo imaging together with an expanding availability of different biosensor animals promises to enable the further assessment of SNA and its effects in the vasculature of living animals.

Correlative intravital imaging of cGMP signals and vasodilation in mice

Cyclic guanosine monophosphate (cGMP) is an important signaling molecule and drug target in the cardiovascular system. It is well known that stimulation of the vascular nitric oxide (NO)-cGMP pathway results in vasodilation. However, the spatiotemporal dynamics of cGMP signals themselves and the cGMP concentrations within specific cardiovascular cell types in health, disease, and during pharmacotherapy with cGMP-elevating drugs are largely unknown. To facilitate the analysis of cGMP signaling in vivo, we have generated transgenic mice that express fluorescence resonance energy transfer (FRET)-based cGMP sensor proteins. Here, we describe two models of intravital FRET/cGMP imaging in the vasculature of cGMP sensor mice: (1) epifluorescence-based ratio imaging in resistance-type vessels of the cremaster muscle and (2) ratio imaging by multiphoton microscopy within the walls of subcutaneous blood vessels accessed through a dorsal skinfold chamber. Both methods allow simultaneous monitoring of NO-induced cGMP transients and vasodilation in living mice. Detailed protocols of all steps necessary to perform and evaluate intravital imaging experiments of the vasculature of anesthetized mice including surgery, imaging, and data evaluation are provided. An image segmentation approach is described to estimate FRET/cGMP changes within moving structures such as the vessel wall during vasodilation. The methods presented herein should be useful to visualize cGMP or other biochemical signals that are detectable with FRET-based biosensors, such as cyclic adenosine monophosphate or Ca²⁺, and to correlate them with respective vascular responses. With further refinement and combination of transgenic mouse models and intravital imaging technologies, we envision an exciting future, in which we are able to “watch” biochemistry, (patho-)physiology, and pharmacotherapy in the context of a living mammalian organism.

Ca(2+) signaling in arterioles and small arteries of conscious, restrained, optical biosensor mice

Two-photon fluorescence microscopy and conscious, restrained optical biosensor mice were used to study smooth muscle Ca²⁺ signaling in ear arterioles. Conscious mice were used in order to preserve normal mean arterial blood pressure (MAP) and sympathetic nerve activity (SNA). ExMLCK mice, which express a genetically-encoded smooth muscle-specific FRET-based Ca²⁺ indicator, were equipped with blood pressure telemetry and immobilized for imaging. MAP was 101 ± 4 mmHg in conscious restrained mice, similar to the freely mobile state (107 ± 3 mmHg). Oscillatory vasomotion or irregular contractions were observed in most arterioles (71%), with the greatest oscillatory frequency observed at 0.25 s⁻¹. In a typical arteriole with an average diameter of ~35 μm, oscillatory vasomotion of a 5–6 μm magnitude was accompanied by nearly uniform [Ca²⁺] oscillations from ~0.1 to 0.5 μM, with maximum [Ca²⁺] occurring immediately before the rapid decrease in diameter. Very rapid, spatially uniform “Ca²⁺ flashes” were also observed but not asynchronous propagating Ca²⁺ waves. In contrast, vasomotion and dynamic Ca²⁺ signals were rarely observed in ear arterioles of anesthetized exMLCK biosensor mice. Hexamethonium (30 μg/g BW, i.p.) caused a fall in MAP to 74 ± 4 mmHg, arteriolar vasodilation, and abolition of vasomotion and synchronous Ca²⁺ transients.

Summary: MAP and heart rate (HR) were normal during high-resolution Ca²⁺ imaging of conscious, restrained mice. SNA induced continuous vasomotion and irregular vasoconstrictions via spatially uniform Ca²⁺ signaling within the arterial wall. FRET-based biosensor mice and two-photon imaging provided the first measurements of [Ca²⁺] in vascular smooth muscle cells in arterioles of conscious animals.